We respect the copyright so if the Museum site explicitly states in its Terms of Use that their images are their copyright and we need a written permission, we write an email and ask. No means no - we DON'T use them and skip to more friendly museums sites. Happily, there are many of these and every day brings more, because people there unlike 10 years ago now understand that it's a WRONG idea to protect images of the artworks - that are by all means in Public Domain - from being used. "open access" means do whatever you want. Others, non-commercial&educational only, &we say 'ok'.