Many of our students today think it is just fine to take anything they find on the internet to use for themselves. As educators we have to include and model how copyright and fair use work in the classroom. Within this, we have to teach them the importance of following the same rules in their lives indefinitely. No one wants to get arrested or sued because they decided to find an image on the Internet to use in a pamphlet.
Public domain is a set of intellectual property that has lost its copyright in some manner. There are routes to public domain. Copyright expired or the renewal was just not done correctly are the first two. There are some works that are not protected by copyright. And then there are the works that the creator decides to put in the public domain. The easiest way to find out about these works is through Creative Commons. Their website even has a search that can find the works you are looking for that also are part of public domain, which helps avoid the aforementioned legal issues.