Mrs. Silver's Art Gallery
Resources:
Web Gallery of Art. A searchable database of European painting and sculpture from 1100-1850.
Artcyclopedia - Searchable database, a bit of everything and well-organized.
National Gallery of Art for Kids
Metropolitan Museum of Art Timeline - Easy place to search collection. Searchable by artist, time period, subject, etc.
Metropolitan Museum of Art for Kids - Sophisticated interactive studies of selected areas and/or artists designed for kids. 
Explore and Learn is another way to get to the above Met sites but I included it because there are a lot of other portals. The Daily Artwork Archive is worth a click.
Fine Art on Line. This site is designed by Harcourt School Publishers and has interesting biographical material on a few artists, as well as giving students an opportunity to pick out the artist's works from other pictures. Also has  information presented interactively on graphic arts and fiber arts as well as games.
The Art Institute of Chicago
MOMA has two interesting portals. Destination Modern Art is an introduction for young elementary. Art Safari is more sophisticated and explores the theme of animals.
 
Van Gogh Museum in Amsterdam has lots of information. At the top is a black navigation bar. Under the "For Children" section are coloring pages and there are e-cards of the paintings with explanations. For older children, there is a short multimedia presentation about his life (to find it, "Vincent Van Gogh", "Index", it is right at the top of the page "Overview." ) To see pictures, on the navigation bar go to "The Museum "and lclick on "Permanent Collection." An excellent site for looking at the pictures is The Vincent Van Gogh Gallery.
 
Mega-Page of Art-for-kids-links.

Sister's Wendy's Selected Works
African Art:
National Museum of African Art - from the Smithsonian. Lots of stuff.
Make your own mudcloth. This opens the introduction. Click on the box on the right: Make your own Bogolanfini.