We've added a brand new page to our Resources section. Come search for “free” images at various government agency sites. Several websites offer images that are either public domain or have very generous usage rights. Be sure to read usage terms at each site.
We call them “free” as they were paid for with U.S. tax dollars (which means they are now free to use). Start to explore now and find images for your next project (many of these are high-resolution as well!) Please share this page with your friends or students.