Series of questions that will help you determine if you can use a picture and the terms, laws and ethics around using that image.
Images posted openly on the web are not necessarily the public domain.
To find reusable images:
Use Creative Commons -- enter Search term under "Find Licensed Content"
Copyright laws for images : Copyrightlaws.com was initially registered as a domain name in 1998 by Lesley Ellen Harris. It has since been a portal for copyright and licensing information for creators, owners, distributors, and consumers of copyright-protected materials.
Section 108 of the US Copyright Code: Handy spinning wheel chart for libraries and archives regarding section 108 of the US Copyright code.
"Professor Eric Faden of Bucknell University created a humorous, yet informative, review of copyright principles delivered through the words of Disney characters -- an ironic bit of commentary given Disney's key role in lengthening copyright terms."
The video is is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 License.
Ch. 1: Copyright definition
Ch. 2: What things can be copyrighted (2:50)
Ch. 3: Copyright duration and the public domain (3:45)
Ch. 4: Fair Use (6:20)
Ch. 5: Why use Disney cartoons? (8:10)