Some universities maintain frequently-updated image site portals (collections of sites). Although duplication is frequent, these are other sources of information:
Bluffon College - Index to Art-Historical [web] sites - a really great smattering of quality image collections
Boston College - includes "Art on the Web"
Boston University - "Finding Images on the Web"
Sweetbriar College, Virginia - Chris Witcomb's famous "Art Resources on the Web"
Art Museum Sites - Yahoo's updated site of most popular (most frequently used) websites of art museums
University of Michigan's collection of art museum sites
Art Inventory of American Painting and Sculpture Smithsonian American Art Museum, delivers images only for owning entities that have submitted them.
Joconde - French national ministry of Culture's centralized database of art museum images