ARTstor logo. Retrieved from Wikimedia Commons
ARTSTOR Digital Library is a database that contains more than two million images of art, architecture, the humanities, the social sciences, the sciences, and photography which span various cultures and time periods.
Guides available here to get you started
Remember - It is your responsibility to make sure that images you find fall within the public domain. Always check an institution's 'Terms and Conditions' regarding image use.