The Smithsonian Institute has made a large number of photos available for use on Flickr. Most are easily available for educational use. This is an excerpt from the terms of use:
"Permitted Uses
The Smithsonian welcomes you to make fair use of the Content as defined by copyright law.Information on United States copyright fair use law is available from the United States Copyright Office: http://www.copyright.gov/fls/fl102.html. Please note that you are responsible for determining whether your use is fair and for responding to any claims that may arise from your use.
In addition, the Smithsonian allows personal, educational, and other non-commercial uses of the Content on the following terms:
You must cite the author and source of the Content as you would material from any printed work.
You must also cite and link to, when possible, the SI Website as the source of the Content.
You may not remove any copyright, trademark, or other proprietary notices including attribution information, credits, and notices, that are placed in or near the text, images, or data.
You must comply with all terms or restrictions other than copyright (such as trademark, publicity and privacy rights, or contractual restrictions) as may be specified in the metadata or as may otherwise apply to the Content. "
Images from Wired Magazine, with Creative Commons licensing.