If it was a print of the painting, it would depend upon whether a court would construe that it had sufficient originality to be a derivative work. But that wouldn't stop Getty from trying to assert copyright.
http://en.wikipedia.org/wiki/Bridgeman_Art_Library_v._Corel_....
Handheld and video cameras using existing light are welcome in the permanent collection galleries. Please check at the entrance of the changing exhibitions for any restrictions in those galleries. Flash photography, tripods, and monopods are not permitted.
Those limitations make making a good reproduction a challenge.
As to your question: they are probably asserting the copyright on the photograph itself, not on what it depicts. That is perfectly OK. If that weren't possible, press photographers would have a hard time making a living.