Photos for Class
http://photosforclass.com/ - Finding Creative Commons images through a CC Search is a good option for students. But getting students to attribute those photos correctly can add time to the work exponentially. Photos for Class removes that hassle. It automatically adds a black ribbon across the bottom of the image with the appropriate attribution.

Pics for learning
www.pics4learning.com/
"image friendly library" Permissions granted for use and the bibliography info is displayed for each picture right on the page.

Wikimedia Commons
commons.wikimedia.org/wiki/Main_Page
Many thousands of images. Scroll down under full sized image to see the particular licensing for this particular image. Almost all require attribution; some, but only a few do not allow derivative works (which means you can't edit the picture.)

Google Advanced Image Search
http://www.google.com/advanced_image_search
Under Usuage, choose - "free to use and modify"
Whenever the is a person's name be sure to credit them for the image.

FlickerCC
http://www.flickr.com/creativecommons
Photos on Flicker by their Creative Commons licensing. Look for the category, then click on "See more" and search for what you want.

Free Foto
http://freefoto.com/
 "FreeFoto.com is the largest collection of free photographs on the Internet (link back and attribution required)."

Ookaboo
http://ookaboo.com/o/pictures/
Creative commons site for finding great (free) images for class work.

Find Icons
http://findicons.com/
Nice site for free icons.

WPClipart 
http://www.wpclipart.com/index.html
All images are public domain. While originally created for Word Processing and printing, they work well as web images.

OpenClipArt
http://openclipart.org/
All public domain. Very slow server.

How to attribute - always include url:
{{PD}} - public domain
{{CC, by person's name}} Creative Commons, Attribution
{{Car Photo, Joe. T., Lincoln School}} original photo by team member.