Pixabay has a large amount of CC0 photos.

New York Public Library The NYPL has a large image collection and you can easily check a box to limit your search to public domain. These are mostly old photos and historic documents.

Unsplash all CC0 photos, and a nice interface.

Stockography free stock photos.

Flickr General Search with “No known copyright restrictions” for CC0 and public domain. You may also search for “All Creative Commons” and use CC BY, CC BY-SA, or CC BY-ND, provided you follow the license terms. There are also public domain Flickr groups.

Flickr Commons Collection of institutions with publicly available images – be careful though – not all images from all institutions are CC0 or public domain.

The Getty Institute Mostly art and historical images. Search with filter “open content images” .

Wikimedia Commons includes CC0 and public domain but also many more restricted CC licenses – use with caution.

Totallyfreeimages.com All images are in the public domain

Google Image Search Advanced image search allows you to filter by labelled for commercial use, but not by type o f license, so you still have to check the source. It will almost always be more efficient to use a search that allows you to filter by license.