Tip! You can use images produced by the government without having to seek permission or heed copyright.
Search Google Images and limit to .gov
Search example:
HST view of Mars. Source: http://rst.gsfc.nasa.gov/Sect19/Sect19_10.html
OAIster: union catalog of digital resources
American Memory, The Library of Congress
MADID (Yale University Manuscripts & Archives Digital Images Database)
VIA (Harvard University Visual Information Access)
Bridgeman Art Library: art, culture & history images
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