Since the Danish cartoons are in the news again, I stumbled upon the controversial Wikipedia article on "Depictions of Mohammed" and from there to the excellent Zombietime Mohammed Image Archive.
But being the person I am, I wanted to find a heretofore unknown depiction of Mohammed from older times.
I found him in an illustration of a biography of Mohammed by famous American writer Washington Irving, called Mahomet and his Successors. The books seems to have been written in 1849; this edition was published in 1869:
Google Books hasn't yet censored this one.