I'm not sure how to best describes it, but it's basically the sort of feeling of instinctual uneasiness/repulsion some people will have in face of very lifelike automatons or such.
A wiki here might explain it better:
http://en.wikipedia.org/wiki/Uncanny_valley
In a way, it would be a testament to the realism of a photomanipulation if it can ellicit such a reaction however.