Copyright Friendly Images

With grateful thanks to Christy DeMeyer, Teacher-Librarian at Golden Senior High School for building this list :-)
NO Citations/Attribution Typically Required
Open Clip Art Library - http://openclipart.org/
MorgueFile http://www.morguefile.com/
U.S. Government Photos http://www.usa.gov/Topics/Graphics.shtml - Vast majority are public domain but a few links direct you to sub-sites where you will see a disclaimer limiting use.
Pics4Learning - http://pics4learning.com/
PicDrome - http://www.picdrome.com/ (Public Domain pictures)
Flicker Commons - http://www.flickr.com/commons -
Alegri Photos- http://www.alegriphotos.com/
Burning Well - http://www.burningwell.org/
Public Domain Images - http://www.public-domain-image.com/
Public Domain Pictures - http://www.publicdomainpictures.net/
Google Stock Images - 5,000 stock images for use in your Docs, Sheets, and Slides. Go to Insert > Image, click on Search, filter to Stock photos, and search for the images that you want.
Free to Use with Credit/Attribution
General
Free Photo Gallery - http://www.freephotogaleries.com/
FreeFoto - http://www.freefoto.com/index.jsp
Creative Commons - http://search.creativecommons.org/
Wellcome images http://medphoto.wellcome.ac.uk/ (medical & social images)
Wikimedia Commons - http://commons.wikimedia.org/wiki/Main_Page
Open Photo Project - http://openphoto.net/ (blocked)
Picture Collection Online (New York Public Library) http://digital.nypl.org/mmpco/

History Focus
Graphic Maps.com http://www.graphicmaps.com/clipart.htm
Great Images in NASA (GRIN) http://grin.hq.nasa.gov/
Images of American Political History http://teachpol.tcnj.edu/amer_pol_hist/
America From the Great Depression to World War II: FSA/OWI 1935-1945 http://memory.loc.gov/ammem/fsowhome.html
American Memory Collection: Library of Congress--click on "Photos and Prints" and check the "Copyright and Restrictions" section for each collection
http://rs6.loc.gov/ammem/mdbquery.html
American Memory Maps Collection http://memory.loc.gov/ammem/gmdhtml/gmdhome.html
Selected Civil War Photographs http://rs6.loc.gov/ammem/cwphome.html
USDA Historical Photos http://www.usda.gov/oc/photo/histfeat.htm

Science Focus
NASA Image Exchange http://nix.nasa.gov/
NASA Multimedia Gallery http://www.nasa.gov/gallery/index.html
NASA Photo Archive http://www.nasa.gov/gallery/photo/index.html
NASA's Planetary Photojournal http://photojournal.jpl.nasa.gov/
National Health Museum (AccessExcellence) http://www.accessexcellence.org/RC/VL/GG/ - great images for science (biology, cell processes)
National Library of Medicine: Images from the History of Medicine http://wwwihm.nlm.nih.gov/
NOAA Photo Library (National Oceanic and Atmospheric Administration) http://www.photolib.noaa.gov/
Public Health Image Gallery http://phil.cdc.gov/Phil/home.asp
Science Images http://www.ase.org.uk/cgi-bin/imageFolio/imageFolio.cgi?direct=Science
Science Images (Earth Zone) http://www.ase.org.uk/cgi-bin/imageFolio/imageFolio.cgi?direct=Earth_Zone