Or his minions might have either de-noised the scanned image, or shrunk it, or reduced the color saturation, or all three, or otherwise processed it in such a way that the image would compress to a reasonably-sized jpg in order to ease web traffic. Maybe they just wanted to make it look pretty. Who knows what they were thinking?
If the image started as a decent high-resolution scan, and any of these things were done, would that in your mind constitute forgery? What if they did enough of these things to the raw scan such that the result would make it impossible for anyone to draw any conclusions at all? Where, on the scale of perfect to useless, does the quality of their published image fall? Where, on the scale of certainty to conjecture, do your conclusions fall with regard to the "scan"?
What you are pointing out here is that simply showing differences in images doesn't prove forgery. Other things, such as normal image processing steps, can create differences. As I have said elsewhere, Polarik hasn't proven that any of the differences he finds (even when they really exist) are the result of forgery. He just says so.