There are no restrictions on using works that are in the public domain, which means you can use them however you want—short of claiming that you created them yourself. Unfortunately, it's not always easy to tell whether or not something is in the public domain. There may be some cases when you know for sure that a work is public domain (for instance, if you find a photo or text you are sure was published before 1924), but for the most part, the best way to find public domain content is to search for it specifically.
Public domain refers to materials for which:
The copyright has expired;
The copyright owner has intentionally and explicitly "dedicated" it to the public domain;
The copyright owner did not follow copyright renewal rules; or
Copyright law does not protect (such as works created by U.S. Government employees during the course of their employment, and works that cannot be copyrighted (such as ideas, common knowledge, data points etc.))