In the method proposed in the paper, the signature is stored in the pixels and ostensibly even survives some compression. So if you made a screenshot of a signed image, the image would still have its signature inside the screenshot, but the screenshot wouldn't have any (new) signature as a whole.

That's how I understood it at least.

I would assume you could make big enough differences in color/contrast/brightness and especially 3D rotation (similar to taking a photo of a screen) would make such a signature unreadable... but I would love to be proven wrong.