Here is a list of royalty free image websites. In all cases you should check the license – some restrict the free usage to personal and not commercial use.
http://www.sxc.hu/ This is my goto
http://www.flickr.com/ For Flickr in advanced search check the “Only search within Creative Commons-licensed content” and any other licence-related options you need before you begin Flickr advanced search
http://browse.deviantart.com/resources/stockart/ Always check licenses here.
http://www.fotolia.com/Info/Images/FreeImages This is the free section of a paid site.
http://www.dreamstime.com/free-photos This is the free section of a paid site.
http://search.creativecommons.org/ Always check the licenses on the results from this website.