The images in the collections below have been made available for educational use. However, to cover yourself, please ensure you consult the terms and conditions for each collection.
Flickr Commons: http://www.flickr.com/commons
Library of Congress Prints and Photographs Online Catalogue: http://www.loc.gov/pictures/
The Rijks Studio – from the Rijks Museum: https://www.rijksmuseum.nl/en/rijksstudio
NASA: NASA Images | NASA
National Gallery of Art: https://images.nga.gov/en/page/show_home_page.html
A growing collection of scientific images that spans the scientific, technical and medical fields. Create an account and search the images. Images obtained from SpringerImages can be used for almost all noncommercial purposes, including integration into presentations and PDF documents.
Wikipedia Commons: http://commons.wikimedia.org/wiki/Main_Page
Wikipedia Featured Pictures: http://en.wikipedia.org/wiki/Wikipedia:Featured_pictures