Britannica Image Quest 
Image Quest offers over 2 million easy-to-use, high-quality images from more than 50 international collections, including the British Library, Getty, Oxford Scientific and National Geographic Society. A broad range of subjects are covered, including architecture and design, industry and production, the sciences, environment and ecology, and commercial design. All images have been rights-cleared for non-commercial and educational use, so they can be used for school projects without fear of copyright infringement.

ARTstor (Images)  
More than 1 million Digital Images in the arts, architecture, humanities and sciences for teaching and research. The license allows you to use the images in your presentations and coursework with a relevant citation.