Tracking Changes on Digital Graphics & Videos
A computer security analyst, Neal Krawetz, can analyze digital graphics and videos to determine if it is an original or if it has been manipulated. He can identify the last tool to touch the file, e.g., Adobe PhotoShop or the make/model of the original camera. Using the metadata of the image, he can tell which areas of the photo/video have last been altered. From the article:
Error level analysis involves re-saving an image at a known error rate (90%, for example), then subtracting the re-saved image from the original image to see every pixel that changed and the degree to which it changed. The modified versions will indicate a different error level than the original image.
The Wired article provides some examples of photographs, before and after using this program.