ARTstor and Camio are two databases that are available for reviewing art images. If you register for an account in ARTstor and have an ipod touch, iphone or ipad you can access a mobile version that includes a flashcard application that will test your knowledge of famous works of art.
If you want to gather a basic, but subtantial history on a work of art, an artist or an art movement, check out Oxford Art Online. This work was formerly known as the Groves Art Encyclopdia. It is like the wikipedia of art, but 10 times better.