Artists Rights Society
Copyright and license monitoring organization. Great reference to use if you need to ask about image usage.
Asking For Permissions
Columbia University guide to asking permission to use a copyrighted work from the rights holder. Advice on contacting the rights holder and writing and letter.
Boston University Research Guide for Copyright
Very helpful compilation of resources on copyright and image use.
CAA
(College Art Association) Fair Use Guide.
Creative Commons
Useful information of usage, sharing and remixing works. Many images online and in Wikipedia are covered under a specific CC license. You can use this resource to find out if you can use an image.
Grad Student Guide to Copyright
from the University of Michigan.
Public Domain Sherpa
Guide to use of Public Domain and Copyrighted work.
US Copyright Office Compendium for Visual Art Works
Covers the copyright for different kids of visual art works.
Visual Resource Association Statement on the Fair Use of Images for Teaching, Research, and Study
Six uses of copyrighted still images that the VRA believe fall within the U.S. doctrine of fair use.