Oxford Art Online provides web access to the entire text of The Dictionary of Art, ed. Jane Turner (1996, 34 vols.) and The Oxford Companion to Western Art, ed. Hugh Brigstocke (2001). Articles, and over 1,500 thumbnail art images and line drawings, extensive image links, and sophisticated search and navigation tools.
Ancient World Image Bank through Flickr is a collaborative effort to distribute and encourage the sharing of free digital imagery for the study of the ancient world.
ARTstor contains nearly 1.4 million images in the areas of art, architecture, the humanities, and social sciences. Most (but not all!) of ARTstor's images are of archival or museum materials.