Originally published on Lorelle on WordPress and used here with permission. This list is for my classes and workshops needing public domain, royalty free images for their projects.

Colorful brain scan of a mouse.
Just because you select a filter for public domain and royalty free images on a Google Image search does not mean these images are actually public domain and royalty free. Check. Ask first, they might say yes.
Flickr is a major resource for images, but many of these have restricted use. Not every image on Flickr or the web is free to use. Check thoroughly for usage rights, and if in doubt, get permission or don’t use the image. Check out Flickr’s Public Domain Photos’ Photostream for images specifically marked as public domain.
The images featured range from people, places, wildlife, and space, as well as textures, patterns, and artistic graphic images. These free photos sites listed may have nude or violent images, though I did my best to exclude image sites which specialize in those.
One of the best, kept up-to-date lists for images and graphics in the public domain for free use is Wikipedia:Public domain image resources. I usually start my search from their list, which includes government collections and resources, special subject-based collections, and more.