Recently I became aware of the Hackerfactor blog, especially the posts related to discovering image manipulation. It is interesting to read what one can deduce from an image, even when one doesn’t use such “obvious” information sources like image metadata (I say “obvious” because it seems that it isn’t obvious at all for most people – but at least it can sanitized automatically). So here are the links to the tools he recommends:
You might also find this paper interesting.
All in all, the most interesting thing for me was the fact professional image manipulators (ok, I just made that word up, meaning “people who know keyboard shortcuts in Photoshop”) repeatedly re-save the same image in lossy formats like JPEG, thus compounding the loss of quality. Then again, one should never underestimate human stupidity.
Picture taken from Elsie esq.'s photostream with permission.