The Caves at Lascaux
The Caves at Chauvet-Pont-D'Arc (These sites both contain virtual tours of the caves and their world famous paintings).
Don's Paintings, Engravings and Sculptures This site has rather "grown up" language, but there are some stunning photos of cave paintings. Just click on the little ones to see them properly.

Other websites about Caves

Virtual Cave This site has lots of information and photos about the features of caves, like stalagtites and stalagmites..
Spelunking Wonderland: This site has been produced by a group of American 5th Grade pupils (that's about Year 6). There is lots of interesting information. (Spelunking means exploring and finding out about caves - the science of caves is called spelaeology - bet you didn't know that before!)
Terrestrial Cave Animals Information about the kinds of animals that live in caves.
Nova Caves Choose the 2 sections about Jewel of the Underground and How Caves Form.