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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.
Our PD uses VeriPic for photo management. They have authentication, audit trails, etc. They can detect if a photo has been changed from the camera level. They've been pretty good so far and have held up in court. I don't know all of the case law but their website is www.veripic.com I hope that helps.