Why have a photo of the bishop in the sacristy? In many (most? all?) Catholic sacristies, there is a photograph of the diocesan bishop, and often one of the Pope.

Why?

(Authoritative sources preferred to anecdote.)
Because from the earliest times, bishops have been seen as the fulcrum of the local Christian community: the priests being their fellow workers; and the deacons their servants. Apostolic succession also comes through the bishops who alone can ordain new bishops and priests.
For example, St. Ignatius, bishop of Antioch in A.D. 107, writes in his epistle to the Smyrnaeans (Chapter 8):

See that you all follow the bishop, even as Jesus Christ does the Father, and the presbytery as you would the apostles; and reverence the deacons, as being the institution of God. Let no man do anything connected with the Church without the bishop. Let that be deemed a proper Eucharist, which is administered either by the bishop, or by one to whom he has entrusted it. Wherever the bishop shall appear, there let the multitude [of the people] also be; even as, wherever Jesus Christ is, there is the Catholic Church. It is not lawful without the bishop either to baptize or to celebrate a love-feast; but whatsoever he shall approve of, that is also pleasing to God, so that everything that is done may be secure and valid.


So while there is no explicit or authoritative documentation on the custom, this is almost certainly the reason sacristies have a photo of the local bishop: showing the centrality of the local bishop, and the authority and jurisdiction which comes from him, to and via the priesthood, to the faithful.
In the Roman Rite we also have the acknowledgement of the local bishop in the Canon (a for all intents and purposes unchanged part of the perennial Roman Rite of Mass):

...
To you, therefore, most merciful Father, we make humble prayer and petition through Jesus Christ, your Son, our Lord, and bless these gifts, these offerings, these holy and unblemished sacrifices, which we offer you firstly for your holy catholic Church. Be pleased to grant her peace,  to guard, unite and govern her throughout the whole world, together with your servant N. our pope and N. our bishop, and all those who, holding to the truth,
hand on the catholic and apostolic faith. Remember, Lord, your servants N. and N. and all gathered here, whose faith and devotion are known to you. For them, we offer you this sacrifice of praise—or they themselves offer it—and all who are dear to them: for the redemption of their souls, in hope of health and well-being, and paying their homage to you, the eternal God, living and true. In communion with those whose memory we venerate, especially the glorious ever-virgin Mary, mother of our God and Lord, Jesus Christ,† and blessed Joseph, her spouse, your blessed apostles and martyrs, Peter and Paul, Andrew, (James, John, Thomas, James, Philip, Bartholomew, Matthew, Simon and Jude; Linus, Cletus, Clement, Sixtus, Cornelius, Cyprian, Lawrence, Chrysogonus, John and Paul, Cosmas and Damian) and all your saints; we ask that through their merits and prayers, in all things we may be defended by your protecting help. Through Christ our Lord. Amen. ...

The bishop (as well as the pope) serve to symbolize—as well as realize and maintain—unity and catholicity in the communities that make up the one body of the Catholic Church.