While some school work could be covered under the fair use exemption, it is a good practice to find open access materials to use.
Smithsonian Collections Search Center and Open Access Images –
An online catalog containing most of Smithsonian major collections from museums, archives, libraries, and research units, with over 1.3 million images, videos, audio files, podcasts, blog posts and electronic journals. The Open Access images can be used without attribution. Use in your homework or presentations with no guilt.
Smarthistory –
Conversational videos and essays covering art and cultural heritage from the paleolithic to the present. All resources are published under a Creative Commons non-commercial license and are ad-free.
Art Institute of Chicago –
Explains how to find and use open access images in the institute's online collection.
Library of Congress and American Memory –
The Library of Congress digital collections includes American Memory, American Folk Life Center, National Screening Room, and many more collections presenting texts, sound recordings, still and moving images, maps, and sheet music that document the American experience. "Rights & Access" notes are attached to records.
Creative Commons –
Directly search over 500 million images, use meta-search for audio and video. All content is under Creative Commons licenses or in the public domain. Read license for reuse permissions.
