Google Images is considered to be one of the most comprehensive sources of images available. This site has images that are in the public domain as well as copyrighted by their creators.You can limit your search to images in the public domain by using the following tools to limit your search results.
Access the Google Image web site at http://images.google.com in a web browser.
Search using the keyword(s) that best describe what image you would like to find.
Open the Search Tool to active the Usage rights options. Limit your search by using either
Labeled for noncommercial use with modification or
Labeled for noncommercial use
The Advance Image Search offers the user the option of searching by size, color of image, type of image, domain, and other limits. Access the Advance Image Search here or type in www.google.com/advanced_image_search?hl=en in your browser
To access images that can be used without restrictions choose either:
Free to use or share
Free to use share or modify
Choose one of the following to insure that the image is free from copyright restrictions