"The Pakistani government blocked access to the social networking service Twitter on Sunday, after publicly holding Twitter responsible for promoting a blasphemous cartoon contest taking place on Facebook, officials said. ...
'The material was promoting a competition on Facebook to post images of Islam’s Prophet Muhammad,' said Mohammad Yaseen, chairman of the Pakistan Telecommunication’s Authority, was quoted as saying. He was also quoted as saying that Facebook had agreed to allay the concerns of the Pakistani government."
One of the possible reasons why it is "haram" to make images of Mohammad is that he never existed. In the case of Christ we have accounts of his life by four people who actually knew him but the earliest accounts of Mohammad's life date from around 200 years after he was supposed to have lived.