In 2003, the Los Angeles Times ran a picture by staff photographer Brian Walski of a British soldier in Basra, Iraq, motioning to a man carrying a child. When an astute journalist at the Hartford Courant, one of many newspapers that reprinted the photo, noticed that it seemed to contain repeated images of the same person in the background, the veracity of the picture came into question. Walski admitted that he had used Adobe's Photoshop software to combine two separate photographs for the final image, and was promptly fired.

The Walski episode not only led to a widespread discussion of ethics in photojournalism, but also demonstrated how easily a skilled user can employ programs like Photoshop to fool average viewers -- and sometimes even experts -- into taking a faked image for the truth. Because almost all digital photos, including those used as evidence in court, are vulnerable to this kind of tampering, computer scientists and others are busy advancing the state of the art in digital forensics.

"The problem [of photo altering] had been in my head for a couple of years," says Nasir Memon, a computer scientist at Polytechnic University in Brooklyn who specializes in image processing. He began to see more and more articles in newspapers about digital photo tampering, Memon explains, and decided about two years ago to use his experience in enhancing photos to detect digital alterations.

There's already one way to prevent tampering -- or at least expose it -- but the process is expensive and not widely available. Cameras equipped with "digital watermarking" technology can append an extra stream of identifying data to the image file. If the photo is changed at all, the digital watermark is corrupted. Canon, for one, has been selling cameras with this technology and the supplementary software for reading watermarks since 2002. They're being purchased mainly by professionals, such as crime scene investigators, who need to prove that the photos they take are unaltered.

However, not all photographs used in court are taken by experts using watermarking technology. That's where digital forensics comes in, says Memon, who is organizing a February 2006 symposium on the field in San Jose, CA. The technology can uncover well-hidden alterations in photos taken with regular digital cameras, match an image to the camera that captured it, and determine whether two images were taken with the same camera.

One example is software Memon developed to characterize a brand of camera, such as a Sony or Canon, by its digital "signature." "These are not the things that will help you nail down a criminal," Memon explains, "but clues that form a piece of a puzzle that can solve a crime."