Stock image sources

Copyright free images 

Wikimedia Commons —  Licensed under creative commons; attribution and non-commercial use are required.

Library of Congress Division of prints and photographs —  many but not all are copyright free; anything before 1923 is public domain, and of course anything by the government, including the FSA collection

US Geological Survey  —

US Dept. of Agriculture   — Agricultural Research Service photos

US Fish and Wildlife Service

US Government — general collection of photos

National Oceanographic and Atmospheric Administration

National Aeronautics and Space Agency

Centers for Disease Control 

Also useful: Shutter Muse – free photo  manual