Why is Christ often shown with long hair in Christian iconography? A friend of mine is Catholic and she has an icon or picture of Christ. It shows him with long, dark brown hair. Is there anything in the New Testament that justifies this representation? It seems to me, and I could be wrong here, that it a common representation.

If not, how far can we trace back this representation? Did it become prominent early on in Christianity?

In the interest of clarity, I should point out that I'm a Sunni Muslim and that I do realise that many forms of Islam frown on figurative representation. I'm not really interested in that debate. The question is purely a question about Christian iconography.
It is true that Our lord Jesus Christ is most often portrayed as having long hair. In the New Testament, no reference is made about the outward appearance of Jesus. St. Paul himself was not a fain of long hair as one can read in 1 Cor. 11:14-15.


  14 Does not the very nature of things teach you that if a man has long hair, it is a disgrace to him, 15 but that if a woman has long hair, it is her glory? For long hair is given to her as a covering. 


Let us remember that St. Paul was a Roman citizen and the Romans had shorter hair.

The men at the time of Jesus definitely had long hair and the Jewish Encyclopedia bares this out.


  Fashion Among Men.
  
  From the Old Testament it may be gathered that it was customary for the men to have their hair cut from time to time. The Nazarites allowed theirs to grow uncut for religious reasons. Absalom, proud of his thick head of hair, had it cut once a year only. But generally the hair was cut oftener. It was never shaved save on special occasions; the high priests and the priests in general were expressly forbidden to have theirs shaved. They were neither to shave their hair according to heathen custom, nor to allow it to grow uncut like that of the Nazarites (comp. Ezek. xliv. 20). There is no other information in the Bible concerning the care of the hair.


Not all images of Jesus have Him with long hair, but all modern ones do. In fact the first images found in the Catacombs of Rome have Jesus portrayed with short hair.



Rome, Catacombs of Priscilla – The Good Shepherd


  The Good Shepherd in the catacombs. One of the images represented the in the art of the catacombs is the Good Shepherd. While the model is taken from pagan culture, it immediately takes on a Christological significance inspired by the parable of the lost sheep. Christ is thus represented as a humble shepherd with a lamb on his shoulders as he watches over his little flock that is sometimes made up of only two sheep placed at his sides. - THE CHRISTIAN CATACOMBS  


But why did this imagery of a Christ with short hair to a Christ with long hair come about? Well, the men in Jesus' time historically had long hair. There may have also been other factures involved as to why we only see Christ having long hair in Christian iconography. Among these are the Shroud of Turin, the Veil of Veronica, the Manoppello Image and so on, all of which are seen with Jesus as having long hair.


Shroud of Turin



Veronica holding her veil, Hans Memling, c. 1470



Manoppello Image