This is part of the Wikipedia umbrella project where users can use any of the sites photos, audio, videos, and other media freely. You can search by numerous criteria including topic, location, and source.

With a database of more than 22 million freely usable media files to which anyone can contribute, Wikimedia is a great website for finding different kinds of content. Most of the content can be used or modified by you as long as you attribute the author.

Wikimedia has an extensive list of categories, from astronomy to engineering, and there is even a public domain category to allow you to  filter your results. When using search, in the top right-hand corner, try entering “PD” before your search term to limit the results to public domain images. An exmple would be , search for “PD Bugs Bunny” to show only public domain images of Bugs Bunny in the public domain.