A work is in the “public domain” if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner. See: Cornell University Library's, Copyright Information Center for more on public domain in the United States.
How do I search for copyright free images on Google?
1. Enter a search term in Google Images search.
2. Click on "Tools" then select "Usage rights"
3. Use the drop down menu to select free-to-use, share, and more...