Museums today are in a state of rapid change. They are changing due to our changing landscape in the digital domain. As a result, many museums are making their content available in an open and free way to those who view their web sites. Some museums have NO limit to how the images are used, while others have some restrictions, such as unlimited noncommercial use. To find out the specifics, you will need to read the fine print.
To assist you in this effort, I am including a list of links to a variety of museums world-wide who have open content. A sampling of them:
http://www.metmuseum.org/ (Use acronym OASC for the copyright-free stuff)
http://www.museum-id.com/idea-detail.asp?id=389 (Rijksmuseum)
http://digitalcomicmuseum.com/ (COMIC books!)