Your photos are copyrighted the moment they are "fixed in a tangible medium." There is no requirement for you to register a copyright to enjoy the protections that copyright law provides. Registration is required if you want to sue someone violating your copyright and go after "statutory damages."
The U.S. Copyright Office at http://www.copyright.gov/ has a link that appears to allow registration of a copyright directly.
Lots of info on this document: http://copyright.gov/circs/circ01.pdf. It says this about registration...
Edit: It looks like online registration may be as cheap as $35.00
If registration is made within three months after publication
of the work or prior to an infringement of the work,
statutory damages and attorney’s fees will be available to
the copyright owner in court actions. Otherwise, only an
award of actual damages and profits is available to the