[quote=Nita]It may have something to do with the Shroud of Turin image
and/or the Image of Edessa (aka the Holy Mandylion). According to tradition, this was was a holy relic consisting of a square or rectangle of cloth upon which a miraculous image of the face of Jesus was imprinted.
BTW, a question along the same lines as “Why is Jesus always shown with long hair?” is, “Why do all portraits/icons of Mary look nearly alike?” The answer to this is, undoubtedly, traceable to St. Luke the Evangelist. St. Luke is the patron saint of (among other things) artists, at least in part because St. John of Damascus (7th-8th century) identified him as having painted the Virgin Mary’s portrait. (Tradition holds that St. Luke painted it on the top of a cypress-wood table that came from the home of the Holy Family. At the request of the faithful, Mary sat for the portrait. When it was finished, she was pleased, saying that “My grace shall accompany it.”)