Another nice resource for public domain images: Pixabay. Property and model releases are still required for commercial work. More here. Buying photos from a stock photo site comes with model and property releases, but if you’re just making projects for yourself, and nothing commercial is involved, sites like Pixabay are a treasure trove of fun stuff.
There lots of great images at Pixabay, from cute critters (though way, way too many spiders!!) to some stunning landscapes. I also saw a V3 Mermaid and I’m pretty sure this old PC set in an image too. 😛
Edited to add thumbs of just a few images I downloaded: