Flickr Trick: Finding Free-to-Use Images details the steps to take to find Creative Commons licensed photos.
To find these photos on Flickr, go to Flickr’s advanced search page and enter your search criteria. Scroll down to the bottom of the search page and check the box that says “Only search within Creative Commons-licensed photos.” If your site is monetized by ads (even just Google Adsense), subscription access (even just for archives), or other means, then you probably want to play it ultra-safe and also check the “Find content to use commercially” box as well. Also, if you plan to crop, montage, or otherwise manipulate or remix found images, check the “Find content to modify, adapt, or build upon” box.