The Bosch attribution is part of the bizarre world of international copyright, and I don't know what law should apply to the photograph (which is surely *NOT* 75 years old). But the moral point I was making is that the original work of art is not just floating in the internet free to use, but is in a museum and appears to be owned by the Spanish "Patrimonio Nacional". Someone made a photograph of the artwork, and that photographer is not named. (The documentary nature of the photograph is one of the elements complicating any question of copyright, as is whether the photographer would retain copyright or the museum, and whether US or UK law would apply.)

The museum put a digital image of the artwork on their website, but that doesn't mean the digital image is free to copy (it may be, but putting it on the website does not in itself make it so--in fact, there is a copyright notice at the bottom of every page on the website).