Leonard Evens , Mar 20, 2004; 10:03 a.m.
I am fairly confident that it is possible, using the current public/private key cryptography methodology, to introduce additional information in an image which would allow anyone to detect a forgery and even to recover the original image. Certainly a digital signature can be added to an image in encoded form which any manipulation would destroy. If the signer tells us how, such a signature can be read by anyone but not modified without destroying the signature and possibly the image. I don't believe however that many images produced by present day technology incorporate such features and I don't think there are as yet any standard ways to do it.
Now and in the immediate future, I think you have to examine an image carefully. A lot of digital editing, even that meant to mislead, is done clumsily and careful examination of the image will demonstrate it. A good example is the widely circulated photo in which images were combined to show a young John Kerry standing next to Jane Fonda. It seems clear to me from the perspective and subtleties in the lighting that the image is a forgery.