Teachers and students can legally use copyrighted materials without the author’s permission if it falls under “fair use” guidelines.  Section 107 of the 1976 Copyright Act states: “The fair use of a copyrighted work, including such use by reproduction.... for purposes such as criticism, comment, news reporting, teaching, .... scholarship, or research, is not an infringement of copyright.”
From Teaching with Digital Images, page 43, International Society for Technology in Education., 2005. 

This information is not to be considered legal advice.  When in doubt about copyright issues, consult with your building administrator.

• Motion Media - 10% or 3 minutes - whichever is less
  
• Text Material - 10% or 1,000 words, whichever is less
  
• Individual Author - 2,500 words or 250 words (poems)
  
• Music - 10% but not more than 30 seconds from a single work
  
• Illustrations & Photographs:  5 images by an artist or photographer in one multimedia program
  
• Published collection:  Not more than 15 images
  
• Database Numerical Sets:  10% or 2,500 fields or cells of entry

For a more detailed list see:  Fair Use Guidelines for Educational Multimedia  Published by the Consortium of College and University Media Centers

Crediting Sources

PLEASE NOTE:  The images and documents  in these collections are in the public domain. Though you may not need to ask permission to use them when publishing on the Web for educational purposes, you still must cite these images unless otherwise notified.  If you see any copyright notices on these pages, read them for further instructions.