"Pics4Learning is a safe, free image library for education. Teachers and students can use the copyright-friendly photos and images for classrooms, multimedia projects, web sites, videos, portfolios, or any other project in an educational setting.
The thousands of photos in this collection are approved for use in the classroom and indexed and sorted in order to maximize their effective use by students in a 21st century classroom.
While the photographers retain the rights to these photos, they have graciously allowed their use through this collection."
The web site makes it easy to search by topic and sub-topic.