Compfight (http://www.compfight.com/) was recommended to me by a couple of people at the Open University.  It searches images on Flickr and has an option for Creative Commons images. Next to the search box you can switch between Creative Commons Only, Creative Commons Commercial and Creative Commons Off. The results are displayed as thumbnails and you simply click on an image to go to the original image page on Flickr.

This is a very quick and easy way to search and preview Flickr photos with Creative Commons licenses, but do check the license of the image on its original Flickr page. There are several CC licenses with different levels of permissions and for all but one of them you are required to acknowledge the photographer.