Removing the fish from the tank is the only way that I know of to conclusively prove that perspectives aren't being played with, and the fish is the size advertised. Actually, I imagine a plastic baggie is not bad - the fish cannot thrash around and hurt himself, and he is angled and held straight, meaning that the photo can be taken very quickly and then the fish released. The smooth surface of plastic should be more gentle on the slime coat than a wet paper towel.
They aren't going to be lingering over this process. These peoples' livelihoods are depending on them providing healthy, quality fish as advertised.