It finally occurred to me to type “public domain” into the Wikipedia and see what came up. Lots of things, that’s what.
Among them I found a mother lode of sources of public domain images. So if you need art a Web page you’re working on and want to be free and clear, or for some other project, there’s the place to go.
The Library of Congress’ American Memory site is especially helpful. Key in whatever you want, and let it search. If the photo was taken by or for a government agency or is older than 1924, you can use it in your own work.
Note that panning and zooming on a still in a video project can be extremely effective. It’s a trick I employ all the time in order to avoid showing a talking head.