The U.S. Copyright Office defines "public domain" as a work "...no longer under copyright protection or if it failed to meet the requirements for copyright protection." The public domain also includes works where the creator has freely released the work into the public domain.
On the web, lots of people freely release their copyright in order to expose their talent and their work as widely as possible. Anything -- content, software, images, fonts, videos, etc. -- in the public domain may be used freely by anyone without contacting or gaining the permission of the originator.
Finding Images and Content in the Public Domain
1. Searching for "public domain" along with various words or phrases that you are seeking is an easy enough way to find free images and content on the web.
Looking for an image of Abraham Lincoln? Google "public domain image Abraham Lincoln," and you'll find the photo of President Lincoln shown above. (The Library of Congress has been kind enough to upload it for us.)
2. Wikipedia has also provided an online list of links that provide free, public domain images -- a list that is continually growing. It is an excellent resource. (One of my favorites is Wikimedia Commons.)
3. Again, Wikipedia provides a long and growing online list of links providing free, public domain content -- worldwide. It includes links to huge public domain content collections such as Project Guttenberg.