ArtSTOR offer high quality images for your educational use. After creating an account you can download high resolution images and create presentations.
Oxford Art Online also has an image database.
Many museums and libraries will offer online exhibits and digital collections:
The Getty: online images: http://www.getty.edu/art/gettyguide/
The Brooklyn Museum:http://www.brooklynmuseum.org/opencollection/artists/564/Aztec