I always pondered on the idea of embedding a few bytes of watermark in an image color channels for the same purpose, storing minimal info in an invisible way for the viewer.
For instance a naive blurry idea approach would be to encode a relative difference in the R color channels of all 4 corner pixels of the bitmap.
It could get a bit more robust or sophisticated as well of course.
I am aware compression and crop could corrupt this but it would be feasible to add redundancy or workarounds up to a certain point for these issues.
Any idea if this has been done before ?