I hesitated posting this because a lot of the surrounding information is technical, so technical that even I didn’t understand all of it. However, I think this information is so interesting it is worth putting out there and letting you decide what it is worth to you.
As I understand it, here’s one method some of the experts use for figuring out if a picture has been digitally offered. This PDF written by Neal Krawetz describes the process of analyzing JPGs in a couple of different ‘spectra.’ These spectra are actually an analysis of the error level of an image.
When you save a file in the JPG format you are losing some of the data. Different quality JPGs mean that more or less data from the original image has been saved from the file. During the process of making edits to an image, the material that is inserted is almost always at a different error rate than the orignal file that existed.
This, however, is not something that would be readily recognizable to the naked eye. There is a program that will allow you to analyze the different levels of error by implementing it on a heat map. Take a look at the link for an example. This program is pretty simple to operate simply open the program. Click on file –> open. Select your picture, and then adjust the bottom bar to a few different locations. It may take a couple of different settings to be able to find the right setting to get the best view of it. Finally, concentrations of unusual color will be suspicious, and are the most likely areas to be altered.
I should mention that I did have some stability problems with this program on Windows Vista, so I’m not sure if that’s a problem with the operating system or the program. It is something to keep in mind.
So beware when you alter that image, most likely someone out there can call you out on it, and now so can you.