Public Domain works are not protected by copyright law and they may therefore be legally used without permission.
A work may enter the public domain when/if:
More info: Copyright Term and the Public Domain in the United States. (January 2013 Cornell University) and Is it in the Public Domain?, a series of flowcharts from Samuelson's clinic at UC Berkeley Law School.
While certain works may be in the public domain, expressions or versions of that work may be under copyright protection.
Read and comply with the use restrictions for specific image sources and always cite images someone else created.
[Image: Lee, Russell, Sonoma County, California.
The noon collection of eggs..., 1942.
Library of Congress.]
Creative Commons licenses provide a way for people to share their work and make it available for others to build on and reuse.
Although Creative Commons content won't cost you any money to use, it's not totally free; to use it, you must follow certain rules. People who choose to make their content Creative Commons can choose one or more of these licenses to apply to their work:
Use a Creative Commons License for your own images.
Search Creative Commons (images, films, musics, etc.)