I highly recommend the search engine at creative commons. You can search for works with various creative commons licenses. I generally use ones that are free to use for commercial work due to my ads. i don't care so much about share alike and attribution as I am happy to do those things. Creative Commons has explanations of all of the license types.
Choose a license will explain the license types. Explore has the search engine stuff.
Another good site is open clip art.
Hubpages help files has a very useful page on using images and will tell you how to site the sources.
I reference that page a lot. it also talks about creative commons some more.