Don't be an idiot. It is, in fact, trivial to prove beyond a reasonable doubt -- just sit the alleged perp in front of a computer and see what he can "create" from scratch.
That's exactly how a Dutch painter, accused of helping the Nazis steal the nation's art treasures, proved that he was merely guilty of forgery.
Second, it will never fly in a court of law, because the police must prove that it is an actual child. The perp does not have to prove he has the skill to create a digital image.