Three Suggestions
CanStockPhoto (http://www.canstockphoto.com/) offers over 15 million professional royalty free stock photos and give out free weekly downloads. They have tens of thousands of new images added every week and their generous license agreement gives great flexibility.

Fotosearch (http://www.fotosearch.com/) allows users to search from more than 32 million images. They can also find royalty-free clip-art pictures, illustrations, stock video clips, poster, prints, and more. Their database contains images from over 120 stock images vendors from all around the world.

GoGraph (http://www.gograph.com/) offers over 18 million affordable royalty free stock photos, illustrations, vector clip art and royalty-free footage clips. Their database contains images on a wide variety of topics and can be viewed for free or purchased for any project.
