Some years ago I read the book The Public Domain about copyright and learned an interesting tidbit: photos of public domain paintings or photos are not covered by copyright in the U.S., they’re free to reuse.
Here’s the relevant bit from Wikipedia:
Reproductions of public domain works
The requirement of originality was also invoked in the 1999 United States District Court case Bridgeman Art Library v. Corel Corp. In the case, Bridgeman Art Library questioned the Corel Corporation‘s rights to redistribute their high quality reproductions of old paintings that had already fallen into the public domain due to age, claiming that it infringed on their copyrights. The court ruled that exact or “slavish” reproductions of two-dimensional works such as paintings and photographs
Another court case related to threshold of originality was the 2008 case Meshwerks v. Toyota Motor Sales U.S.A. In this case, the court ruled that wire-frame computer models of Toyota vehicles were not entitled to additional copyright protection since the purpose of the models was to faithfully represent the original objects without any creative additions.
The wire-frame case is obviously relevant to computer graphics. There’s a rundown of other countries’ laws on Wikimedia Commons’ site.
Private collections are within their rights to limit access as they wish, as misguided as I think it is to sell public domain works to the public. I have a problem with any public institution invoking protection of photos of works, since there’s no legal basis for this.
The Public Domain is free to download and worth a read. To be honest, after a bit I skimmed chapter 2, but I particularly enjoyed chapter 7, a case study in which the U.S.’s more permissive rules on what is in the public domain (“sweat of the brow” works are not copyright in the U.S.) are contrasted with Europe’s more restrictive laws.
Oh, and if you like to read about copyright (you weirdo), you might enjoy The Idealist: Aaron Swartz and the Rise of Free Culture on the Internet. The second half is worthwhile, though quite sad, and a story I suspect many of us know to some extent. The first half is about the evolution of copyright laws in the United States, which went from being a haven for piracy of foreign (primarily English) works to an ardent defender of extending copyright almost into perpetuity (despite there being no incentive benefits in extending copyright retroactively, since the law at the time the work was created was found sufficiently appealing to the original author; sorry, I feel a rant coming on…). Ahhh, imagine this alternate universe. <– That’s a great link, by the way, well worth a click.