An interesting analysis of a graphic recently used by Victoria's Secret in their advertising. This gives chapter and verse of the techniques used, and results obtained, demonstrating the ability to determine if an image has been altered, and even which parts of an image have been modified, and how.
I find this particularly interesting because of the apparently widely held belief that steganography is "undetectable" without comparision to the original image. Most of the "Photoshop disasters" are glaringly obvious to the naked eye. As this demonstrates, analysis and detection of modification is easily accomplished, even when the differences are not apparent to the human eye. (Well, except for the straps. That was pretty stupid ...)