He became a bit of a pop phenomenon, with stores dedicated to selling high quality prints of his works.

And one of the things, so I'm told, that could happen, was that you could buy one of his prints, but someone at the store, an artist, would dab some paint onto the print. Add some "light" to it.

Obviously this is a commercial endeavor, so I doubt the provenance of the rights holder was in doubt (such as through an employment agreement).

But it's, perhaps, an interesting case study about who owns what in a time of augmented and manipulated media.