Draw a cartoon of anyone. Start by exaggerating the features. Make his ears bigger, his nose longer, stretch out his jaw, push out his eyes and if your picture is wearing a turban, watch the bullets fly.
Islam frowns on images of its frowning prophet, not to mention images of anything else, but it is particularly offended by the institution of the cartoon. The photo keeps us human. The painting can even enhance and flatter its subject. But the cartoon reminds us of our flaws.
And Mohammed, despite his child-molesting, caravan-robbing, sex slave-keeping and occasional demonical possession ways (see Satanic Verses; The, the subject of yet another murderous censorship Jihad), is considered the perfect man. And anyone who disagrees must die.