The only place we had problems taking pictures was the Mona Lisa at the Louvre. It's absolutely forbidden there. Although people tried to take pix. I hope the guy I stepped in front of enjoys his picture of my back (hehehe).

Flashes are a serious issue with paintings. Although your one flash might not do any damage, when you multiply it by several hundred thousand per year...besides, it can be annoying to be in a museum like the d'Orsay and have flashes going off.

My wife did a good job with our Sony digital sans flash in the Louvre and d'Orsay. Both museums are well-lit.