Reference Pictures  (reference.pictures)
I didn't find this in the list and looks like an incredible resource:

http://reference.pictures/

Some of them are paid. Some of them are free.
Some you can set the price you are willing to part with or if you want it for free (if you can't afford or with your usecase doesn't really makes sense to pay).

The way the site works is that there are articles with some selected previews which represent the collection. A collection can have 100-700 photos at high resolution (e.g. 6720x4480px) and the sizes are in the gigabytes.

Per FAQ, licensing is very open but no resell or repackage allowed:
> “What can I use these photos for?”
> 
> Just about anything you want. Use them for personal or commercial projects without any attribution required (though it’s always appreciated if you like what we’re doing).
> 
> You are not allowed to resell or repackage these photos, though. Because that’s a dick move.
> 
> Most folks use them to study art, as elements in matte paintings, or to send postcards to their grandmothers. Whatever works for you.

