By virtue of appearing to be a photograph. The medium itself is a message (cf. Marshall McLuhan). An image which appears to be a photograph conveys a meaning to

By virtue of appearing to be a photograph. The medium itself is a message (cf. Marshall McLuhan). An image which appears to be a photograph conveys a meaning to the viewer that "this depicts a real event", whereas if I draw a picture, even if it is of something plausible but which as never happened, there isn't a perception that the image is being presented as evidence of an event which has taken place. If we know that enough people will draw such an inference from a statement, it can and should be treated as potentially libelous.