Web Gallery of Art "A searchable database of European painting and sculpture of the Gothic, Renaissance, and Baroque periods (1200-1700), currently containing over 15,400 reproductions." Biographies, commentaries, guided tours are available. Provides good-sized images with detailed file information.
World Images "The internationally recognized WorldImages database provides access to the California State University IMAGE Project. It has just been selected by the Library of Congress for inclusion in its historic collection of Internet materials. It contains approximately 80,000 images, is global in coverage and includes all areas of visual imagery." This site is great for images of Asian art.
Participating institutions (including the Library of Congress, the Smithsonian, and the George Eastman House) have contributed their photography collections to Flickr Commons, and you can add tags or comments to these images.