The Digital Image Rights Computator is a tool to determine if an image is in the public domain and may be helpful to you.
http://www.vraweb.org/resources/ipr/dirc/page_1.html
You might try to contact the website where the image was found.
I would suggest that you limit use of images to those in the public domain. Sites like pics4learning.com are the most fool proof.
Public Domain Images is another example.
Public-domain-image.com
You can also search the Creative Commons for images with licenses that allow educational use.
Creativecommons.org

http://newtoolsworkshop.wikispaces.com/Fair+Use+and+InfoEthics