Clinamen

A strict reading of the 'constitution' of the wikipedia actually says there probably shouldn't be any images in the Muhammad article. The reason is that the references in the Muhammad article are completely absent of any images, in fact they are disclaim it pretty vigorously. The only way to find images for Muhammad is to specifically search for them, but that's a deliberate act- there's no policy that says you have to have images in an article; so the images refer to Muhammad, but Muhammad sources don't link to the images. But the article is supposed to be on Muhammad not the images.

Almost certainly the real reason is simply the largely Christian and Jewish contingent of the site's editors and in particular the administrators.