from the author (Score 3, Informative)

In response to some of the posts on our work, we have developed a number of different techniques for detecting certain types of tampering. For those interested in technical details, please see:

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ih04.html
sacv03.html

And, we have two new papers currently in review (abstracts are currently on-line, and preprints will be available soon):

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sp05b.html

Some of these techniques work, as some have pointed out, only on high-quality jpeg or uncompressed images, while others work on lower-quality images. We are only in the early stages of development, and are currently working to extend some of these ideas to low-quality jpeg and gif images (though this will likely be a harder problem given that the compression artifacts will overwhelm any statistical perturbation resulting from tampering). One outcome of this may be that a legal standard is set that enforces images brought into a court of law to be of a certain resolution and compression quality.

I will be the first to admit that each of the techniques that we have developed can be reverse-engineered, though doing so is more difficult for some techniques than others. It is our hope, however, that as we and others continue to develop more techniques it will become increasingly more difficult (though never impossible) to simultaneously foil each of the detection tools.