I agree with many who mentioned
This is a great place with many useful photos, especially for those who design for education and science. You won't be able to find this elsewhere. The prices are very cheap (25-50 cents), so I wonder how these guys run their business so well. And yes, if you play well you can get photos for free.
One more site which is new on the market:
slightly higher prices but many exclusive photos.
Also worth mentioning is
http://www.istockpro.com - this is a high end photography site with many low priced fine images.
Good luck! http://www.istockphoto.com/user_view.php?id=362802