The following sites will be useful in locating free images and media:

FreeFoto "Non-commercial users may download web size images to use off-line in school projects, church services, cards, leaflets, etc. Basically if your off-line use is not commercial you can download our web size images for free."
Pics4Learning Copyright-friendly images for educational use.
Free Photo Bank
Library of Congress Photos through Flickr
Morgue File Free photo archive.
NOAA Photo Library
Public Domain Images

Smithsonian Flickr images
NASA Images