According to John Woods, Islamic history professor at the University of Chicago, it was not simply the depiction of Muhammad that was offensive, but the implication that Muhammad was somehow a supporter of terrorism.

Not all Islamic traditions ban images of Mohammed, and some are pretty lax about pictures of lesser figures. For Muslims, the rule against depicting God and the prophets comes from the Hadith, a collection of sayings and actions attributed to Mohammed. (A few passages in the Quran have been taken to offer oblique support to this notion.) The doctrine has been interpreted in various ways over the centuries. Persian art of the 15th and 16th centuries shows the figure of Mohammed with his face, hands, and feet covered. Some earlier Persian works show full views.

Arab Muslims tend to be the strictest about religious imagery. Shiites are more flexible than Sunnis; for example, they display images of Husayn, * the grandson of Mohammed. Devotional portraits of leading teachers are generally OK, as long as they don't fall under the Hadith ban on depicting the major prophets. Pictures of people in religious scenes—like pilgrims on the Hajj—are also allowed. These are more likely to be displayed in the home than at a mosque, and some conservative Muslims will refuse to pray in their presence.

Muslims are more or less unanimous on the subject of Allah—he can't be drawn under any circumstances.

Or did Larson draw another Mohammed, and not the prophet? There's so many of them, it's bound to lead to confusion.