Public Domain is FREE to use without restrictions! You can find images, music, books, and more in the Public Domain. You can use creative works that were published before 1923. There are some exceptions, but to maintain best practices you will want to stick with pre-1923 works.
Remember, Even though these works are without restriction... You should ALWAYS cite your resources!
"Public Domain Infographic." Viintage (http://viintage.com/public-domain-images/) - permission to use as long as linking back to Viintage.