summary: user uploaded photos of public domain works, and the impression i get from comments on slashdot, is that in the uk they'd have a case because the photos are copyright of photographer but since this is a US user and US servers, the user will be fine due to Corel v Bridgeman (http://en.wikipedia.org/wiki/Corel_v._Bridgeman) which decided that photos of public domain stuff is not copyrighted.
Flash photography, as they are worried about the cumulative effect of millions of flashes per year damaging the pigments, same as leaving the painting out in the sun would.
/All/ photography in the National Portrait Gallery in London is forbidden, with or without flash. They are worried about losing control on works where copyright has already expired.
They claim copyright of the photographer. IANAL but don't think they have a case. Otherwise you could copyright a Shakespeare play based on the time you spent typesetting its text.
There are multiple copyrights associated with most content items.
You don't get copyright based on time nor the effort involved - rather it is based on originality of the work.
So you can't claim the copyright of a play by typesetting it - but by typesetting it one can have the copyright in your original typographical arrangement of that play.
So just because the underlying work is in the public domain does not mean it safe to copy a representation of it which has been created in a way which allows the person who created the representation to claim copyright over their work.
Compare a person selling a photocopied 17th century print of a book against a person who retypes the text of that book and lays it out in an original way.
IMHO the issue here is with the policies of the publicly funded museums - there is an argument that if a public funded organisation creates a new copyright work based on public domain material then they should place that new work within the public domain or at least license it using CC.
There is a doctrine in UK copyright law known as "sweat of the brow". I imagine this is what the NPG's lawyers are referring to when they mention "painstaking effort":
looks as if things are going to be k.