The copyright laws of the United States, which require actual creativity, would seem to argue that photographs of paintings are not derivative works in themselves - no new copyright is created, and the only copyright in question is that of the original work. After all, the aim of the copier is to reproduce the painting as accurately as possible. A perfect copy should have no changes whatsoever; such change as there is are undesirable errors, not creativity.
There is some, but not much, case law in this area. The Corel Corporation was involved in a lawsuit that established some precedent here, where it was decided that copying digital images of out of copyright works was not a violation. Some argue that this is not a good precedent since the plaintiff admitted that the digitization was "slavish"; they argue that this admission should not have been made. However, others argue that the slavishness of the reproduction was self-evident and indeed the precise intention of the reproduction of the painting.
The laws in other nations differ in what qualifies for copyright protection. In this matter, treaties like the Berne Convention offer little help, since US courts have ruled that the scope of what is copyrightable as it applies in the US even for works of foreign origin is entirely defined by US law.
-Matt (User:Morven) who is entirely not a lawyer. As with all free advice, this may be worth only what you paid for it.