"The old Islamic ban on depictions of the prophet, though frequently ignored as these many paintings show, began to gain increased authority in the 18th century. Handmade books of history, especially those painted to legitimize one or another Islamic dynasty, were seldom commissioned after that.
"'The growing power of conservative faculties in Islamic universities also strengthened the old ban on depictions of the prophet,' As'ad AbuKhalil said. 'So did the rise of the Wahabis in Arabia. Their conservatism went so far that they obliterated the prophet's tomb.' They feared its veneration.
"But theological taboos can evolve. Sometimes they tighten, sometimes they loosen."
And this from the Iconography entry on Wikipedia:
"Icons are strictly forbidden in Islam if they are consecrated. However Islamic art differs in its view of icons ranging from totally forbidding drawings and photography as with the Wahhabis to forbidding only drawings but not photography to allowing both as with the majority of Sunni Muslims. Some Shia allow even the depiction of the Prophet Muhammad and his cousin Ali, a position totally unacceptable to the Sunnis. Muslims view sanctified icons as idols, and strictly forbid the worship of an icon, or worship in front of it."