I'll quote the positive answer given to me by Cornellia Cornelius in case of future similar inquiries:

Dear Gisela,
It really is a thorny issue or can be. If you're talking about a Botticelio, I think you'd be okay since the painting is in public domain. While its true that the museum does own the painting, most of the time, they don't actually own the copyright, well not usually. Now, if you're talking of using a photo though, I would ask permission from the owner of the photo.