This important gallery for Renaissance Art has indexing by artist and work. Biographies of artists, illustrations and short histories of the works are available. The interests of art historian Jacques-Edouard Berger are well represented in the Berger Foundation's collection of 100,000 slides. The site is in Switzerland, so response is slow. ArtLex Short Definitions for 2,600 terms, plus almost 50 short essays.
Some definitions have visual links, as do most of the essays.
Larry is a photography and art instructor at TCC and Skagit Valley College. This is his useful list of links to photography and art sites.