From Time magazine:
Sometimes, a photo is simply too good to be true. Tiny details in an image, for instance, may be too similar to have occurred naturally, suggesting a cut-and-paste maneuver. Or the color patterns may be too “normal” — beyond the limitations of sensors on digital cameras.
A growing number of researchers and companies are looking for such signs of tampering in hopes of restoring credibility to photographs at a time when the name of a popular program for manipulating digital images has become a verb, Photoshopping.