I have read just about 0% of the posts in this thread, so if this point was made, ignore me:
They are mad that Muhammad was depicted and especially mad that he was depicted with a bomb on his head. They feel, and understandably so, that the image depicts all Muslims are terrorists.
So how do they respond?
Death threats, telling Europe to "learn a lesson from 9/11", storming embassies, Palestinian gunmen briefly kidnapping a German citizen, protesters in Pakistan chanting