"Muhammad at the Ka'ba" from Siyer-i Nebi (1595). Muhammad is shown with a veiled face.
from the Wikipedia article on “Aniconism in Islam”:
“Aniconism is the avoidance of images of sentient beings in some forms of Islamic art. Islamic aniconism stems in part from the prohibition of idolatry and in part from the belief that creation of living forms is God's prerogative. Although the Quran does not explicitly prohibit visual representation of any living being, it uses the word musawwir (maker of forms, artist) as an epithet of God. The corpus of hadith (sayings attributed to the Islamic prophet Muhammad) contains more explicit prohibitions of images of living beings, challenging painters to "breathe life" into their images and threatening them with punishment on the Day of Judgment. Muslims have interpreted these prohibitions in different ways in different times and places. Religious Islamic art has been typically characterized by the absence of figures and extensive use of calligraphic, geometric and abstract floral patterns.
However, representations of Muhammad (in some cases, with his face concealed) and other religious figures are found in some manuscripts from lands to the east of Anatolia, such as Persia and India. These pictures were meant to illustrate the story and not to infringe on the Islamic prohibition of idolatry, but many Muslims regard such images as forbidden.”
