I’m afraid I don’t know the original source – it looks to have been a Frog (this *is* offend everyone day, isn’t it?) – oh hold, on, it is Plantu. I’ve delicately altered the original, I bet you can’t tell where :-).
Predictably enough this offends the wackos which I presume is the point – to try to tire them out. Also predictably enough, there is a wiki page on this: [[<a href="http://en.wikipedia.org/wiki/Everybody_Draw_Mohammed_Day"Everybody Draw Mohammed Day]].