Because it would prove that there was one man greater than Mohammad! That is the crux of the religion that there was no man greater than Mohammad. A man dying and rising from the dead would trump a child molesting murderer as the true prophet of God wouldn't it?
A picture of a guy being crucified who wasn't Jesus, but only looked like Jesus, would prove there was one man greater than Mohammed?
[sigh]
Doesn't anyone get a joke around here?