I’m adding Wikimedia Commons to The Best Online Sources For Images.
It has four million images, and their reuse agreement states:
almost all may be freely reused without individual permission according to the terms of the particular license under which it was contributed to the project. Depending on what you want to do with it, you probably do not need to obtain a specific statement of permission from the Licensor.
Seems about as broad as you can make it…
I use Wikimedia commons extensively with my aquaculture work because I can quickly find images that I’m allowed to use that I often can’t find though other sources.