Two sites I use a lot for royalty free images are: http://commons.wikimedia.org/wiki/Main_Page and http://www.flickr.com/creativecommons. I especially like the Wikimedia Commons site because it has an easy search feature where you can look for photos by subject matter. You are free to download, edit, remix and use images from Wikimedia without worrying about running afoul of copyright laws.The Flickr Creative Common page is similar, though you often need to browse more through the images before finding exactly what you are looking for. Many images on this site also have an attribution requirement, but it is fairly easy to drop a link back to the photographer's Flickr CC page. I hope this helps!