Here's an interesting interview with Hany Farid, Proving That Seeing Shouldn't Always Be Believing:
Q. Why do scientists need to know about this?
A. Because not long ago, researchers from South Korea had to retract papers published in Science because the photographs used to prove that human stem cells had been cloned were effectively Photoshop-cloned, and not laboratory-cloned. There have been other recent cases, too. And today, in science, more and more, photographs are the data. The Federal Office of Research Integrity has said that in 1990, less than 3 percent of allegations of fraud they investigated involved contested images. By 2001, that number was 26 percent. And last year, it was 44.1 percent.
Mike Rossner of The Journal of Cell Biology estimates that 20 percent of the manuscripts he accepts contain at least one figure that has to be remade because of inappropriate image manipulation. He means that the images are not accurate reflections of the original data. Rossner estimates that about 1 percent of the papers have some piece of image data that is downright fraudulent.