Nicholas Wade has an article in the Times about faking photography. I thought this was interesting:
"He and Dr. Rossner plan to add software tests being developed by Hani Farid, an applied mathematician at Dartmouth. With a grant from the Federal Bureau of Investigation, which is interested in ways of authenticating digital images presented in court, Dr. Farid is devising algorithms to detect alterations.
His work has attracted interest from many people, he said, including eBay customers concerned about the authenticity of images, people answering personal ads, paranormal researchers studying ghostly emanations and science editors.
For the latter, Dr. Farid is developing a package of algorithms designed to spot specific types of image manipulation. When researchers seek to remove an object from an image, such as a band from a gel, they often hide it with a patch of nearby background. This involves a duplication of material, which may be invisible to the naked eye but can be detected by mathematical analysis.
If an object is enlarged beyond the proper resolution, Photoshop may generate extra pixels. If the object is rotated, another set of pixels is generated in a characteristic pattern. "