The Chronicle of Higher Education has an article about the problems of digital image manipulation. It's a very tough thing. I've had several pictures that are clear through the microscope, but the subtleties are hard to capture on film. A little contrast enhancement makes it more visible, but have you changed the data in a way that changes the interpretation?
I'm going to have to think about whether and how to talk about his in my biological writing class this fall.
If you want to see professional retouching in a way that will make you look twice at images for a while, try looking at this professional retoucher's web page.