If wondering if published works, such as images, photographs, songs, selected text, or books, are in the public domain or not…

As found on the U.S. Copyright Office website:

Works Originally Created on or after January 1, 1978

A work that was created (fixed in tangible form for the first time) on or after January 1, 1978, is automatically protected from the moment of its creation and is ordinarily given a term enduring for the author’s life plus an additional 70 years after the author’s death. In the case of “a joint work prepared by two or more authors who did not work for hire,” the term lasts for 70 years after the last surviving author’s death. For works made for hire, and for anonymous and pseudonymous works (unless the author’s identity is revealed in Copyright Office records), the duration of copyright will be 95 years from publication or 120 years from creation, whichever is shorter.

According to Cornell Copyright Information Center, works published in the United States before 1923 are in the public domain and works published from 1923 through 1977 if published without a copyright notice are in the public domain as well.

According to another great resource for information on copyright, fair use, and public domain:
http://fairuse.stanford.edu/Copyright_and_Fair_Use_Overview/chapter8/8-a.html#1

Thousands of works published in the United States before 1964 fell into the public domain because the copyright was not timely renewed under the law in effect at that time. If a work was first published before 1964, the owner had to file a renewal with the Copyright Office during the 28th year after publication. No renewal meant a loss of copyright.

If you plan on using a work that was published after 1922, but before 1964, you should research the records of the Copyright Office to find if a renewal was filed.

See Starting Your Copyright Research

Also:
Standford.edu:

Because of legislation passed in 1998, no new works will fall into the public domain until 2019 when works published in 1923 will expire. In 2020, works published in 1924 will expire and so forth.

They have more information here.

You can search the US Copyright Office databases for the copyright registration of books, music, films, sound recordings, maps, software, photographs, art, multimedia, periodicals, magazines, journals, newspapers and ownership transfer information by visiting:
http://www.copyright.gov/records/