The Journalist's Resource's guide to copyright-free images provides tips on how to locate free photos and images.
Wylio Searches only Creative Commons content in Flickr. Additional tools for blogs and websites, including resizing tool, photo credit builder, and code generator.
Wikipedia Disclaimer: "The presence of a resource on this list does not guarantee that all or any of the images in it are in the public domain. You are still responsible for checking the copyright status of images before you submit them to Wikipedia."
Please read the policy on image use and etiquette at: Wikipedia:Image use policy.
There are no copyright restrictions on "public domain" images. The public domain includes images created by government agencies, and works where copyright has expired (those created prior to 1923).
Images posted on the Internet are not necessarily in the the public domain. To find images that you can "reuse":