Most images in the subscription databases mentioned here are available for use in educational contexts without any worries about copyright. For other sources, use of images for education contexts may fall under "fair use" or may require permission from the original owner of the photograph. This fair use guide from MIT has more information about using images in compliance with copyright law.
These resources can help you find high-quality images of existing artworks that you can use for classroom research and presentations. Access is provided through WIU Libraries.
These image resources are freely available online, but may have more restrictions on their usage. They may not be limited to just works of fine art.