Fair Use is built into the copyright law to address the tensions between the rights given to the copyright holder and freedom of speech. If you want to use copyrighted material without requesting permission from the copyright holder, you must engage in a four factor Fair Use analysis.
1) Purpose and character of the use
2) Nature of the copyrighted work
4) Market Impact
Aufderheide and Jaszi, in their book Reclaiming Fair Use, argue that while you need to ask yourself all four questions to conduct a Fair Use analysis, the courts have shown they are most interested in the answers to the following three questions (24).
Tufts' policy is that it is up to you to decide if the use of the images may be fair or not based on the Four Factor analysis, however, help is available!
The Creative Commons (CC) can be a great tool for copyright holders and individuals seeking copyrighted materials to work with. Copyright holders who want people to use their work can choose among a number of different licenses (from just attribution to attribution-non-commercial-no derivatives). And for people looking to work with copyrighted materials without having to do a Fair Use analysis (search Flickr for Creative Commons images), the Creative Commons can be a great solution to use and re-use creative work.