Due to the high availability of public domain images, the Images category only shows these, as there's no need to look elsewhere or use more restrictive licenses. All of these are Creative Commons Zero (CC0) or otherwise public domain.
- Public Domain Archive
Curated by one person, but with submissions from many, this site has some stunning high-res photographs.
10 new photos every 10 days. Every 10 days, the crew at Unsplash posts the 10 best submissions they've received since the last post. This means there might not be as many as other sites, but they're high quality.
Similar to previous entries in this list, Pexels curates photos from user uploads or public domain images found elsewhere. At least 70 new photos are added every week. The interface of this site makes it stand out. It's very usable and has some great info about many of the images.
Gratisography is different from the others on this list because all images were photographed by one person. You might think that makes it less useful, but the photographs are stunning, often quirky, and can provide some visual interest to your projects.
If you need pictures of food, this should be your first stop. The photos are all taken by the same person, but there's plenty of variety here. Be sure to take a look at the licensing FAQ, because while you're allowed to use the photos in any way you want, it's not CC0, and there are some minor restrictions only related to selling and reselling photos.
There are countless other places to find images without restrictions, but these
are my favorites. If you can't find what you're looking for on this list, here
are two other lists by others that can help.
WP Tavern's list of 13 sources for Public Domain/CC0 images
Sellfy's list of 43+ Unique Places to Find Remarkable Free Photos