Photos For Class has thousands of pictures and artwork and even creates the citation for you!
Creative Commons is especially good if you want to modify someone else's pictures, or use pictures in something you want to sell. You will need to copy and paste the citation.
This site has clip art that you can use for school projects. The creator, Phil Martin, has said that you don't have to cite them.
If you still can't find what you need, try. using Google Image Search. Use this link to go to the Advanced Search page.
Just remember to:
- Scroll to "Usage Rights" and choose "Free to use or share."
- Scroll to "SafeSearch" and choose "Filter explicit results."
- Cite your source. (Ask for help if you need it!)