I was surfing the web for some artwork to include in an upcoming project, and ended up finding this really great repository of public domain artwork:
It’s such a great site. There are some fantastic pieces in there that would be great for gaming, like these creepy double-exposed photos of child laborers (great ghosts for a horror game), these pages from this 18th century manual of black magic, or this 1775 compendium of demonology.
Seriously, there’s way more than that. Give it a good browse.