> ...a photo agency that makes stock photos available on its website with copyright watermarks can markpaint them in such a way that anyone using common editing software to remove a watermark will fail; the copyright mark will be markpainted right back. So watermarks can be made a lot more robust.

I fail to see how this can actually combat against someone with the patience to manually paste out copyright marks using something like a clone brush, or even a normal brush ?

If someone wants to steal your work, they will. There's no sure-fire way to stop them.