Each image you find at the sources listed below may have specific requirements for it's legal use. Academic use does not equal FREE use. For example, the owner may require payment, a link back to their site, or attribution to the photographer. Please read the license agreements closely for all images you acquire online.
Public Image Reference Archive.
Free Public Domain Photo Database
Even though most of these images are in the public domain be sure and read the license with each to make sure there aren't any restrictions.
NOAA Photo Library
National Oceanic and Atmospheric Administration Photo Library.
U.S. Fish & Wildlife Image Library
Sponsored by U.S. Fish & Wildlife Services - a public domain library of related images.