I'd say "The best things in life are free" but that wouldn't be accurate here. More appropriately, "Why pay twice?" The federal government has several agencies that document their work and make it available to the public. This work was paid for with tax dollars, and the people of the United States own the work. Fortunately, through the Internet, the U.S. government is willing to share a lot of it with the world.
Be sure to check out the following Web sites. They offer printresolution images that you can use. Nearly every image is copyright-free, but you may be required to cite the source. Be sure to look at the terms of use posted on the site. Be sure to fully explore each site; you'll be surprised by the wealth (and diversity) of available images:
Center for Disease Control-Public Health: http://phil.cdc.gov/phil/home.asp
CIA World Fact Book: www.cia.gov/cia/publications/factbook
Creative Americans 19321964: http://memory.loc.gov/ammem/vvhtml/vvhome.html
Images of American Political History: http://teachpol.tcnj.edu/amer_pol_hist/_browse.htm
Library of Congress: http://lcweb2.loc.gov/ammem/ammemhome.html
NASA Earth from Space: http://earth.jsc.nasa.gov/sseop/efs/land
NASA Image Exchange: http://nix.nasa.gov
NASA Visible Earth: http://visibleearth.nasa.gov
National Park Service Digital Image Archive: http://photo.itc.nps.gov/storage/images/index.html
NOAA Photo Library: www.photolib.noaa.gov
U.S. Army Corps of Engineers Digital Visual Library: http://images.usace.army.mil
U.S. Air Force Link: www.af.mil/photos/index.asp
U.S. Antarctic Program: http://photolibrary.usap.gov
U.S. Department of State: www.state.gov/r/pa/ei/pix
U.S. Fish & Wildlife Services: http://images.fws.gov
U.S. Government Image Portal: www.firstgov.gov/Topics/Graphics.shtml
U.S. Navy: www.news.navy.mil/view_galleries.asp
USDA Agricultural Research Service: www.ars.usda.gov/is/graphics/photos
USDA: www.usda.gov/oc/photo/opclibra.htm