These online repositories provide high-quality images of artwork and other historically significant items that are in the public domain, so you are free to use them in your papers and presentations, but make sure you cite your source!
Wikimedia Commons contains, according to the site, a collection of over 58 million freely usable media files to which anyone can contribute."Files include images, audio, video, animations, maps, and other multimedia. Users can choose from multiple images sizes when downloading, and each image is often accompanied by information about the work depicted as well as copyright information.
(click on image to enlarge)
If you're searching for an image to republish (on a poster or in a paper, for example), you'll need to make sure you have the right to do so. Copyright holders can use a Creative Commons license to assign usage rules and let users like you know what can and can't be done with images. Use the Openverse (formerly Creative Commons) Search page to find licensed images that you can use for scholarly purposes.