When using images from the web, make sure you are not breaking copyright law. Ask permission from the source or use images that are licensed as Creative Commons. In addition, government images usually fall in the public domain and are free from copyright.
Here is a list of online resources for copyright free images.
Creative Commons Image Directories
A list of Creative Commons licensed image-based media.
Flickr Creative Commons
Many Flickr users have chosen to offer their work under a Creative Commons license, and you can browse or search through content under each type of license.
Flickr: The Commons
A collective of institutions that have uploaded publicly held photography collections. Images found in Flickr Commons site have “No Known Copyright Restrictions."
Google Advanced Image Search
You can limit your results to creative commons by selecting "labeled for reuse" under usage rights.
Wikimedia Commons Images
A media file repository making available public domain and freely-licensed educational media content.Also Bowdoin College has an enormous archive of images. Consult with the library for old photographs, in particular.
Library of Congress Prints and Photographs
This catalog contains photographs, fine and popular prints and drawings, posters, and architectural and engineering drawings.
NASA image library
A collection of over a thousand images of significant historical interest scanned at high-resolution
National Center for Biotechnology Information Image database
Diagrams, charts, graphs and more from full text resources at NCBI..
U.S. Government Photos and Images
A list of image sites for various federal departments.
Also, DigiTales has compiled a list of websites offering free images.