There are many sites now where generous photographers share their work under a CC0 license ( Allowing it to be available to the public domain) . You can spend hours browsing each of these sites to save time, just use these two...
Pixabay scours all the other public domain image sites so you don't have to. Think of it like a mini version of Google just for public domain photos.
If you would want to go straight for the very highest quality, most stunning public domain photos then go to:
Their Images do show up in Pixabay but as a much smaller collection (and so quicker to browse).