Here are some sources to add to your arsenal:
Wikipedia: Public Domain Image Resources
http://www.freeimages.co.uk
http://www.freestockphotos.com/
http://interactive2.usgs.gov/learningweb/fun/image.asp
http://oposite.stsci.edu/pubinfo/pictures.html
http://www.defenselink.mil/photos/
http://www.sxc.hu/
http://earth.jsc.nasa.gov/
http://www.usda.gov/oc/photo/opclibra.htm
http://photo2.si.edu/
http://gallery.hd.org/index.jsp
http://www.freefoto.com/
http://lcweb2.loc.gov/pp/pphome.html
http://holylandphotos.org/
http://www.nara.gov/nara/searchnail.html
http://www.nv.doe.gov/news%26pubs/photo ... ms/atm.htm
http://www.nasa.gov/gallery/photo/
http://www.imagesoftheworld.org/
http://www.photolib.noaa.gov/
http://netvet.wustl.edu/pix.htm
http://www.nps.gov/
Stephanie