Here is the summary:

 I agree that wikipedia articles may have pictures and it is preferable to show them for the convenience of the reader. The pictures of Jesus, Mary etc. are very commonly depicted in the churches and other places, so whenever there is article to explain about Jesus and christianity then it is much preferable to give the abstract idea to the user about how 'Christianity looks like and what is associated with it (including the pictures of Jesus and Mary)'. Now agreeing to the this, the problem is that there are no images associated to Muhammad or other Prophets in Muslims as well as Mosques and any other places. So, when a reader will come to know about Isamic articles then he will be given the idea of 'how Islam looks like and what is associated to Isalm and Muhammad (including Islamic calligraphy)'. Now if there does not exist and prominent association about Muhammad's picture then it is NOT necessary to somehow bring 'Any insignificant' picture no matter it is drawn by some 12 year old kid or some very unpopular artist who is not only unknown in the nonIslamic world but Muslims too.
 So, when a Muslim comes to the article about Muhammad, he will just feel strange on the rediculous depiction and might even laugh that "I did'nt know there is such picture used about Islam". 
 Imagine the misery of a wikipedia article in where the follower of the religion himself has to laugh at what is being shown as a fact about his religion.  
 I hope my comments look valid at the same time when I agree about the value of an image where it is actually represending the article and not contradicting it.