100,000+ people sign petition to have image removed.
For many Muslims, it is forbidden to depict the image of Prophet Muhammad. You might have been aware of this having heard of the massive furor and outrage over a certain Danish cartoon last year.
For Wikipedia, the stated ethos behind their organization is to offer information from a neutral standpoint, with no special discretions given to any particular group of people.
These two tenets have come into conflict recently over the entry on Islam in Wikipedia, which has a photograph of a piece of a 15th century artwork that features Muhammad's face. A Islamic guy who would like to see this image taken down started a petition, and a blog. The petition so far has garnered over 107,000 names.
Wikipedia doesn't seem to be budging, however. This is from a FAQ they had on the matter: "Since Wikipedia is an encyclopedia with the goal of representing all topics from a neutral point of view, Wikipedia is not censored for the benefit of any particular group."
Previous to this incident, Wikipedia also was under some pressure from the German government to remove 1930's era photographs of the Hitler Youth, which is an outlawed organization in Germany in contemporary times. In that case a concession was reached by reducing the amount of images available on the site.