@jollysea @glyph A few more:

https://morguefile.com/ - Just make sure you don't accidentally follow the upsell links.

https://museo.app/ - Searches the collections of a bunch of museums at once.

https://etc.usf.edu/clipart/ - Specializes in old-timey line drawings. Sister site has old maps.

https://www.davidrumsey.com/luna/servlet/view/all?sort=pub_list_no_initialsort%2Cpub_date%2Cpub_list_no%2Cseries_no - Lots of very high res old books and book covers.