As I understand it, in the US, a photograph of a public domain work is itself public domain. That doesn't seem to be the case in the UK, although I think the real legal status is pretty hazy.
The National Gallery makes money selling images of its out-of-copyright paintings. On the one hand, that seems bad, but on the other hand, I'd rather not be paying the whole cost of the Gallery with my taxes.
(And they do have to pay for the very impressive scanning technology somehow.)