University of Florida
Fmnh: Vertebrate Paleontology
Has a large group of vertebrate fossil pictures, a guide on preparing fossils, and much more.
University of Florida
U. Florida Museum: Fossil Horse Cyber Museum
An interesting site about prehistoric horses. Has a large gallery of fossilized horse pictures as well as an area explaining how a horse becomes fossilized.
Other
Lake Powell: Vertebrate Fossils
This site contains several pictures of vertebrate fossils. The text would be too difficult for elementary school but the pictures of the vertebrates could be used to supplement the lesson plan.