As I tell students in my business of photography classes; in the business of photography if you don’t have your copyright, you’ve got nothing.
Register your copyright and keep it.
The way we make money as photographers is licensing the use of our work.
Here’s some information on copyright.
Link to the copyright office website.
Here you’ll find EVERYTHING you need to know about copyright, Including the Copyright Law and how to register your work. http://www.copyright.gov/
Subject to sections 107 through 122, the owner of copyright under this title has the exclusive rights to do and to authorize any of the following:
(1) to reproduce the copyrighted work in copies or phonorecords; (2) to prepare derivative works based upon the copyrighted work; (3) to distribute copies or phonorecords of the copyrighted work to the public by sale or other transfer of ownership, or by rental, lease, or lending; (4) in the case of literary, musical, dramatic, and choreographic works, pantomimes, and motion pictures and other audiovisual works, to perform the copyrighted work publicly; (5) in the case of literary, musical, dramatic, and choreographic works, pantomimes, and pictorial, graphic, or sculptural works, including the individual images of a motion picture or other audiovisual work, to display the copyrighted work publicly; and (6) in the case of sound recordings, to perform the copyrighted work publicly by means of a digital audio transmission.
Source: http://www.law.cornell.edu/uscode/17/106.html
Copyright and Fair Use: http://fairuse.stanford.edu/Copyright_and_Fair_Use_Overview/index.html