The best place to start is in the Creative Commons. This will link you to all the available images through Flickr, Wikipedia and Google. Find it at creativecommons.org and from there you can search for what you need. Licences may vary slightly but you will find all that information there, too. There are a lot more sites that feature free photos as well.
For more information on getting images to use, see this excellent hub: http://hubpages.com/art/royalty-free-stock-photos