Are there rules regarding the content of Roman Catholic artwork? It stands to reason that one cannot display anything that one desires within a Roman Catholic place of worship.  In regards to paintings, statuary, stained glass, and iconography on display withing Roman Catholic facilities (churches, schools, monestaries, etc.), are there rules or guidelines regarding what may or may not be depicted?
For instance, one would never expect to see a statue of the Virgin Mary holding a second infant child.  Obviously this is because Roman Catholicism holds to the perpetual virginity of Mary, but is the dogmatic content of the faith the only rule to follow?  Is the decision left to the individual parish/diocese or are there official rules or guidelines determined farther up the hierarchy?  Where might one find such guidelines?
Are there rules regarding the content of Roman Catholic artwork?
Of coarse there are guidelines, but they are merely to be seen as general norms to be followed. For details about a specific piece of art, one has to try to find a particular decree or pronouncement about it.
Catholic art is art produced by or for members of the Catholic Church. This includes visual art (iconography), sculpture, decorative arts, applied arts, and architecture. In a broader sense, Catholic music and other art may be included as well.
Christian art is nearly as old as Christianity itself. The oldest Christian sculptures are from Roman sarcophagi, dating to the beginning of the 2nd century. As a persecuted sect, however, the earliest Christian images were arcane and meant to be intelligible only to the initiated. Early Christian symbols include the dove, the fish, the lamb, the cross, symbolic representation of the Four Evangelists as beasts, and the Good Shepherd. Early Christians also adapted Roman decorative motifs like the peacock, grapevines, and the good shepherd. It is in the Catacombs of Rome that recognizable representations of Christian figures first appear in number.
In the final session of the Council of Trent in 1563, included a short and rather inexplicit passages concerning religious images, which were to have great impact on the development of Catholic art. Previous Catholic Church councils had rarely felt the need to pronounce on these matters, unlike Orthodox ones which have often ruled on specific types of images.
Images are to be approved of by the local bishop! In cases of doubt one is to proceed to Rome.
The following decree confirmed the traditional doctrine that images only represented the person depicted, and that veneration to them was paid to the person themselves, not the image:

On the Invocation, Veneration, and Relics of the Saints and on Sacred Images
The holy Synod enjoins on all bishops, and others who sustain the office and charge of teaching, that, agreeably to the usage of the Catholic and Apostolic Church, received from the primitive times of the Christian religion, and agreeably to the consent of the holy Fathers, and to the decrees of sacred Councils, they especially instruct the faithful diligently concerning the intercession and invocation of saints; the honour (paid) to [Page 234] relics; and the legitimate use of images: teaching them, that the saints, who reign together with Christ, offer up their own prayers to God for men; that it is good and useful suppliantly to invoke them, and to have recourse to their prayers, aid, (and) help for obtaining benefits from God, through His Son, Jesus Christ our Lord, who is our alone Redeemer and Saviour; but that they think impiously, who deny that the saints, who enjoy eternal happiness in heaven, are to be invocated; or who assert either that they do not pray for men; or, that the invocation of them to pray for each of us even in particular, is idolatry; or, that it is repugnant to the word of God; and is opposed to the honour of the one mediator of God and men, Christ Jesus; or, that it is foolish to supplicate, vocally, or mentally, those who reign in heaven. Also, that the holy bodies of holy martyrs, and of others now living with Christ,-which bodies were the living members of Christ, and the temple of the Holy Ghost, and which are by Him to be raised unto eternal life, and to be glorified,--are to be venerated by the faithful; through which (bodies) many benefits are bestowed by God on men; so that they who affirm that veneration and honour are not due to the relics of saints; or, that these, and other sacred monuments, are uselessly honoured by the faithful; and that the places dedicated to the memories of the saints are in vain visited with the view of obtaining their aid; are wholly to be condemned, as the Church has already long since condemned, and now also condemns them.
Moreover, that the images of Christ, of the Virgin Mother of God, and of the other saints, are to be had and retained particularly in temples, and that due honour and veneration are to be given them; not that any divinity, or virtue, is believed to be in them, on account of which they are to be worshipped; or that anything is to be asked of them; or, that trust is to be reposed in images, as was of old done by the Gentiles who placed [Page 235] their hope in idols; but because the honour which is shown them is referred to the prototypes which those images represent; in such wise that by the images which we kiss, and before which we uncover the head, and prostrate ourselves, we adore Christ; and we venerate the saints, whose similitude they bear: as, by the decrees of Councils, and especially of the second Synod of Nicaea, has been defined against the opponents of images.
And the bishops shall carefully teach this,-that, by means of the histories of the mysteries of our Redemption, portrayed by paintings or other representations, the people is instructed, and confirmed in (the habit of) remembering, and continually revolving in mind the articles of faith; as also that great profit is derived from all sacred images, not only because the people are thereby admonished of the benefits and gifts bestowed upon them by Christ, but also because the miracles which God has performed by means of the saints, and their salutary examples, are set before the eyes of the faithful; that so they may give God thanks for those things; may order their own lives and manners in imitation of the saints; and may be excited to adore and love God, and to cultivate piety. But if any one shall teach, or entertain sentiments, contrary to these decrees; let him be anathema.
And if any abuses have crept in amongst these holy and salutary observances, the holy Synod ardently desires that they be utterly abolished; in such wise that no images, (suggestive) of false doctrine, and furnishing occasion of dangerous error to the uneducated, be set up. And if at times, when expedient for the unlettered people; it happen that the facts and narratives of sacred Scripture are portrayed and represented; the people shall be taught, that not thereby is the Divinity represented, as though it could be seen by the eyes of the body, or be portrayed by colours or figures.
Moreover, in the invocation of saints, the veneration of relics, and the sacred use of images, every superstition shall be removed, all filthy lucre be abolished; finally, all lasciviousness be [Page 236] avoided; in such wise that figures shall not be painted or adorned with a beauty exciting to lust; nor the celebration of the saints, and the visitation of relics be by any perverted into revellings and drunkenness; as if festivals are celebrated to the honour of the saints by luxury and wantonness.
In fine, let so great care and diligence be used herein by bishops, as that there be nothing seen that is disorderly, or that is unbecomingly or confusedly arranged, nothing that is profane, nothing indecorous, seeing that holiness becometh the house of God.
And that these things may be the more faithfully observed, the holy Synod ordains, that no one be allowed to place, or cause to be placed, any unusual image, in any place, or church, howsoever exempted, except that image have been approved of by the bishop: also, that no new miracles are to be acknowledged, or new relics recognised, unless the said bishop has taken cognizance and approved thereof; who, as soon as he has obtained some certain information in regard to these matters, shall, after having taken the advice of theologians, and of other pious men, act therein as he shall judge to be consonant with truth and piety. But if any doubtful, or difficult abuse has to be extirpated; or, in fine, if any more grave question shall arise touching these matters, the bishop, before deciding the controversy, shall await the sentence of the metropolitan and of the bishops of the province, in a provincial Council; yet so, that nothing new, or that previously has not been usual in the Church, shall be resolved on, without having first consulted the most holy Roman Pontiff.

A few cases that I am aware of that have been forbidden by the Church includes those depicting Mary wearing priestly vestments.

First of all I would like to refer to the question of the title of priest attributed to the Virgin in tradition. A writer of the end of the fifth century calls Mary &quot;Virgin, and at the same time priest, and altar who has given us Christ -- bread of Heaven for the remission of sins.&quot;1 After this, there were frequent references to the topic of Mary as priest, which subsequently became the object of theological developments in the 17th century, in the French school of St. Sulpice. In it, Mary's priesthood is not placed so much in the context of a relationship with the ministerial priesthood, but rather with that of Christ.
At the end of the 19th century a true and proper devotion to the Virgin-priest spread, and St. Pius X even accorded an indulgence to its relative practice. However, when the danger was perceived of confusing the priesthood of Mary with the ministerial priesthood, the magisterium of the Church became reticent and two interventions of the Holy Office practically put an end to such devotion.2
After the council, the priesthood of Mary is still spoken of, but it is no longer linked to the ministerial priesthood nor to the supreme priesthood of Christ, but rather to the universal priesthood of the faithful. As figure and first fruits of the Church, she possessed in a personal way that &quot;royal priesthood&quot; (1 Peter 2:9), that all the baptized possess in a collective way. - Mary, Mother and Model of the Priest Fr. Raniero Cantalamessa, OFMCap, Pontifical Household Preacher

Thus images depicting Mary as a priest are now forbidden.

Pope Pius X was originally in favour of this!

&quot;In 1906, Pope Pius X approved a prayer which says &quot;Mary, Virgin Priest, pray for us.&quot; (emphasis mine). However, a Holy Office decree of 1916 forbade the use of any image that had Mary wearing vestments – some say for fear of the possibilty of an argument for women’s ordination, others that Mary as a priest was a metaphorical image taken too far.&quot;- The Priesthood of Mary

The Church sets out that specific colours are to be used within the liturgy for particular liturgical seasons. She also apparently forbids skull and crossbones as well as memento mori or images of the dead (imagines mortuorum) are forbidden on sacred vestments.

Over the years I have shared many photos of black requiem vestments replete with skulls, scythes, bones, even the poor souls in Purgatory. These vestments usually inspire a great deal of interest for reason of their relative rarity and also for reason of these symbols. Reaction to them is either one of great interest or, alternatively, a certain hesitancy and discomfort.
Most of the extant examples of this sort of work comes from the 18th century. In the realm of liturgical books we also find a number examples from the first half of the 19th century. Interestingly, however, such inclusions -- at least as far as paraments were concerned -- were already excluded at least as early as the 1600 edition of the Caeremoniale Episcoporum. The text of the C.E. notes that images of the dead (imagines mortuorum), as well as white crosses (cruces albae), were not permitted on the paraments of the altar, the sacred ministers' vestments, the seldom used missal covering, for the coverings of the faldstool and so on. (See Book II, Chapter XI, No. 1) Later, in the 19th century, clarification was asked of the Sacred Congregation of Rites as to whether the prohibition against images of the dead was meant to only exclude those of the souls in purgatory as opposed to skulls and such, however the answer came back in the negative. It excluded both. (See SRC 4174.)

Here follows three examples of images that are forbidden on sacred vestments:



One last source I would like to add here is the book entitled Matters Liturgical (Collectio Rerum Liturgicarium). This book contains some helpful information, but is dispersed here and there throughout the liturgical norms presented.
Although not forbidden, St. Bernard of Clairvaux was famous for speaking out against gargoyles carved on the walls of his monastery's cloister.

The primary use of the gargoyle was to illustrate evil through the form of the gargoyle,[citation needed] while another theory posits that grotesques in architecture were apotropaic devices. Sometimes the use of the gargoyles illustrated pagan beliefs to reflect the unique cultural history of the community the cathedral is part of. In the 12th century, before the use of gargoyles as rain spouts, St. Bernard of Clairvaux was famous for speaking out against gargoyles carved on the walls of his monastery's cloister:

What are these fantastic monsters doing in the cloisters before the eyes of the brothers as they read? What is the meaning of these unclean monkeys, these strange savage lions, and monsters? To what purpose are here placed these creatures, half beast, half man, or these spotted tigers? I see several bodies with one head and several heads with one body. Here is a quadruped with a serpent's head, there a fish with a quadruped's head, then again an animal half horse, half goat... Surely if we do not blush for such absurdities, we should at least regret what we have spent on them. Gargoyle



A gargoyle on the Basilique du Sacré-Cœur, Paris, France, showing the water channel
I am sure more images have been forbidden by Rome, but I can not locate the particular circumstances.