Creative Commons (one of my favorite non-profits) focuses on issues to do with copyright and particularly is involved in creating copyright licenses that enable you to share your work while retaining some rights (e.g., "do anything you want with it," "use it for free but don't make derivatives," "non-commercial use only," and so on). Their list of main licenses is here:
https://creativecommons.org/share-your-work/licensing-types-examples/
They've now opened an image search engine "for free content in the public domain and under Creative Commons licenses" with "over 300 million images indexed from multiple collections" here:
https://creativecommons.org/share-your-work/licensing-types-examples/
They've now opened an image search engine "for free content in the public domain and under Creative Commons licenses" with "over 300 million images indexed from multiple collections" here:
CC Search Portal
search.creativecommons.org