This article replaces 10 Ways to find illustrations for your writing project.
The following websites are much safer bets, but always check the licence offered and make sure you adhere to its conditions. Even if the licence is CC0, meaning that the picture is free to use and no attribution is required, I think it’s nice to give credit to the photographer or artist, where this is known.
Anyway, here are the sites to explore:
Creative Commons Search Engine (this is the original version, which in my opinion is less flaky and is able to be refined).
British Library — great for old illustrations that are free to use. The collection is being updated continually.
Freepik — this has vector drawings as well.
Pexels — beautiful, hi-res photos.
Pixabay — ditto.
RGB Stock Photos — ditto.
Unsplash — ditto.