One of the best resources for finding images, both current and historical, is the ARTstor Digital Library.
Depending on your research, many other databases may also be useful and are listed below.
Please note that images found in these databases may be under copyright restrictions. Be sure to read the copyright notice before copying, altering, or reposting any media file and review the College Art Association's Code of Best Practices for Fair Use in the Visual Arts.
If you are unsure about how you can legally use a media file, Ask a Librarian for help.