On the left, Chris Foss's "Nemo's Castle", cover for Isaac Asimov's Stars Like Dust. On the right, Glenn Brown's "Ornamental Despair", sold at auction for $5.7m. What's up? Charlie Jane Anders explains at iO9, and quotes the artist himself:
The Foss paintings never look like my versions of them. Mine are always played around with. The colors are altered, the cities were redrawn and I was always inventing things to increase their intensity right from the start. … I never want to lose that notion of appropriation—people say to me, sooner or later you'll stop copying other artists and you'll make work of your own, but it's never been my point to try to do that, because I never thought you ever could. The work is always going to be based on something, and I wanted to make the relationship with art history as obvious as possible.
The argument is that increasing the size and saturation of someone else's work, with some literary and artistic recontextualization, generates a transformative new work—even if the new context amounts to an explicit repetition of the themes already implicit in the original.
There's evidence of at least one lawsuit over Brown's work in the past, but it hasn't reached the courtroom and it's not clear how it settled. For all we know, Foss got a cut.
If you're ever frustrated when someone looks at modern art and dismisses it out of hand, or assumes that the artist and the business are a conspiracy of pretentious idiots and fraudsters, just remember that people like Brown are why.