Pixelfed is a thing, though images don't have to be openly licensed.

I just found openverse, but it's a fundamentally different thing. It's a crawler and search for openly licensed images, but it doesn't host itself. Depending on your usecase that can be an alternative. It has a link to the original site for downloading the pic, but tools like gallery-dl should be able to download it at that point. That means you should be able to find and download images without ever interacting with a proprietary frontend. (didn't try it)