Flickr is a great site with many many great photos and photographers. What is especially nice is that some photographers choose to license their photos under a creative commons license. This Flickr page has a nice explanation about creative commons licensed photos.

If you want to search for creative commons licensed photos, go to the advanced search page on Flickr, then scroll down to the bottom and select “only search within Creative Commons-licensed photos.” Once selected, you can find content that you can use commercially (with proper attribution).

For example, I found this great shot called Surfer at Fort Point by Flickr user Dawn Endico. She licensed the photo under Creative Common’s Attribution-ShareAlike 2.0 license.