Detecting Fake Photos
MIT is working on software that analyzes the surfaces in photos to see if the light patterns all match in order to detect fakes.
Another technique is to check the grain of the photo for irregularities. When an object is placed within a photo, the new object will have differing grain from the original (though there are ways to reduce and eliminate this).
If you're wondering why this matters, check out these examples of how fake photos can be drastically harmful: