ARTstor is the premier resource for finding images for educational use. The images can be used for presentations, lectures and student research papers but not publications.
It contains over a million images from many museums and institutions. Use it without logging in to browse, or create a login to make your own "image groups" that save the images you need.
Visit ARTstor's YouTube site for free videos on using ARTstor, including registering, managing your profile, exporting to PowerPoint, and many more.