Fair Use

FAIR USE - Fair use is a limitation on the exclusive rights of the copyright owner; in other words, it allows reasonable public access to copyrighted works.
The idea is an important part of the English common-law tradition. 

Why does FAIR USE exist?
FAIR USE allows people to use images and written works without compensating the original writer/creator, as long as they act in good faith and do not profit from reproducing the work.  Education, parody, criticism, news reporting, etc are all examples of fair use.
 
The idea of FAIR USE has guided the use of reproductions of works for years.  In most cases, the lack of commercial gain is necessary for a claim of fair use.  If you are a non-profit website designed for educational purposes, you are in the perfect position to claim FAIR USE of images which are already in the PUBLIC DOMAIN.

Copyright

COPYRIGHT - The legal right given to a writer, composer, artist, or a distributor to exclusive production, sale, or distribution of their work. 

Why does COPYRIGHT exist?
It allows artists to profit from their work.  Profit, in turn, creates an incentive to work and distribute the work.

Creative Commons

A set of licences intended to offer the consumer more freedoms than traditional copyright (sometimes abbreviated CC) 

en.wiktionary.org/wiki/Creative_Commons

Creative Commons (CC) is a nonprofit organization that offers flexible copyright licenses for creative works. ... 

www.gitta.info/website/en/html/website_glossary.html

A new "copyleft license" aimed at flexible handling of copyright protection for all kinds of creative work (books, websites, blogs ... 

www.share.uni-koeln.de/

Creative Commons is a non-profit organization dedicated to allowing creators to easily and freely share their work with others through the use of six basic types of licenses. ... 

www.wpi.edu/about/policies/Copyright/glossary.html

Public Domain

The public domain is an intellectual property designation for the range of content that is not owned or controlled by anyone. These materials are "public property", and available for anyone to use freely for any purpose. ...

Why does PUBLIC DOMAIN exist? It exists to allow the free exchange of knowledge.  If it did not, museums would be allowed to keep images under perpetual copyright, thus denying everyone the opportunity to view, critique, or otherwise examine works. Please note that most 20th century works of art are not yet considered part of the PUBLIC DOMAIN. Click here to view a chart of when works pass into the public domain.

en.wikipedia.org/wiki/Public_domain

a work in the public domain is free for everyone to use without asking for permission or paying royalties. The phrase "public domain" is a copyright term referring to works that belong to the public. ...

www.library.ualberta.ca/guides/plagiarism/terminology/index.cfm

The status of publications, products, and processes that are not protected under patent or copyright. 

library.thinkquest.org/06aug/02220/glossary/index.html

Art, literature, photographs, or music that is available for use by anyone, without cost, because the material has not been copyrighted or because the copyright has expired 

www.mbc.edu/grafton/rpc/glossary.php