Did you know? This past spring, Creative Commons launched a search engine that indexes over 300 million public domain images. These are images from 19 image collections and they include works from the Metropolitan Museum of Art, Cleveland Museum of Art, Flickr, and even some CC0 3D designs from Thingiverse.
All of these images are in the public domain or released under Creative Commons licenses, which means they are free to use in a non-commercial setting. It also means they have the licensing information readily available to quick copy and paste.