The Visual Resources Association has released a Statement on the Fair Use of Images in Teaching, Research, and Study. The statement recommends assertiveness on the part of the academic community, and awareness of the guidelines. It concludes that there are six uses of copyrighted still images that fall within the U.S. doctrine of fair use. According to the statement, they are:
1) preservation ; 2) use of images for teaching purposes; 3) use of images on course websites and in other online study materials; 4) adaptations of images for teaching and classroom work by students; 5) sharing images among educational and cultural institutions to facilitate teaching and study; and 6) reproduction of images in theses and dissertations.
You can read the details on these guidelines and more in the full statement.