I'm sure it would be pretty simple to put a simple code in the pixels of the image, could probably be done with offset of alpha channel or whatever, using relative offsets or something like that. I might be dumb but fingerprinting the actual image should be relatively quick forward and an algorithm could be used to detect it, of course it would potentially be damaged by bad encoding or image manipulation that changes the entire image. but most people are just going to be copy and pasting and any sort of error correction and duplication of the code would preserve most of the fingerprint.

I'm a dumb though and I'm sure there is someone smarter than me who actually does this sort of thing who will read this and either get angry at the audacity or laugh at the incompetence.