Sources where you can get interesting and free images:
Getty Museum Collection: https://www.getty.edu/art/collection/
Public Domain Review: https://publicdomainreview.org/collections
Public Work: https://public.work/
PhyloPic: https://www.phylopic.org/
MNAHA Collections (careful, check copyright, not everything is public domain here!) https://collections.mnaha.lu/index/
QIMBY (quality transport): https://qimby.net/
@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.