What Openverse is
Openverse (openverse.org), run by WordPress.org, indexes over 800 million openly licensed images from Flickr, Wikimedia Commons, museums and other sites, with the licence of each.
Best for
Creative Commons photos from Flickr and smaller collections that aren't searched directly here.
Licence and what it means for you
Set per image: CC0, public domain mark, CC BY, CC BY-SA and the non-commercial licences. Commercial-safe searches ask Openverse for commercial licences only.
How Image Asset Server searches it
Through the public Openverse API without a key (anonymous rate limits apply, so results are cached). It re-indexes other sources, so an image also found on Wikimedia Commons is shown once, from Commons.