The most comprehensive annotated list came from Nancy Sharoff. She provided an amazing collection of sites: http://visual-lit.wikispaces.com/Where+Is+It%3F

Also, the following from Colette Cassinelli:
Smithsonian Images
<http://smithsonianimages.si.edu/siphoto/siphoto.portal?_nfpb=true&_pa...>provides

access to that incredible collection, and also allows you to use
any of them as E-Cards. 

Nations Illustrated <http://www.nationsillustrated.com/> has 8,000 images
from around the world, and also provides an E-Card feature. 

The University of Victoria Teaching Clipart
Gallery<http://hcmc.uvic.ca/clipart/>has three thousand images
specifically designed for language-teaching. 

The Royalty Free Clip Art Collection For Foreign/Second Language
Instruction<http://tell.fll.purdue.edu/JapanProj//FLClipart/>from
Purdue University is another place to find images useful for teaching
English Language Learners. 

The Japanese Language Course Support
Site<http://japanese.gatech.edu/images/index.html>is a smaller, but
useful, source of language-learning images. 

Clip Art ETC <http://etc.usf.edu/clipart/> from Florida's Educational
Technology Clearinghouse offers over 38,000 pieces of clip art for students
and teachers. 

Edupics <http://www.edupics.com/> is one more source of clip art for use in
schools.

I'm also adding a direct link to Darren Draper's excellent post (including
additional resources) called The Educator's Guide To The Creative
Commons.<http://drapestakes.blogspot.com/2008/12/educators-guide-to-creative-c...>
http://www.digitales.us/resources/image_sources.php

Here are some others:
From Google: Image Appropriate Sites:
http://www.google.com/coop/cse?cx=011011820386761411814%3Afdioa10ovoi

http://commons.wikimedia.org/wiki/Main_Page

http://commons.wikimedia.org/wiki/Atlas#Selected_map

http://www.wpclipart.com/index.html

http://www.everystockphoto.com/#

http://www.cln.org/themes/clip_art.html

http://openphoto.net/
http://schools.clipart.com /  (not free, but worth looking into for your school site)

http://www.animation-station.com/