A Dartmouth project devised one:
The Dartmouth students and their professor developed a tool and system (described on Mashable) to create a "Photoshop rating" for any given image, so that a heavily edited image would get a five, while something untouched would be a zero. Can you imagine a world in which every advertising image or magazine fashion shoot carried, in effect, a warning level, with a "five" standing for the concept "no one really looks like this?"
More before-and-after photos here.
The Dish 