Put a copyright sign at the bottom though not even technically necessary. According to US law a work is copyrighted upon inception. * Certain exclusions may apply... Not to mention it doesn't really matter whether your site is copyright or not it's easy enough to steal anyones website... Just grab the html, css, images redo it and whip it together. You can always use php, .htaccess files, html encryption, javascript encryption, php encryption, and digimark to protect image files. For the most part though it's very easy to remove protection from images. For example take a screen shot, load it into editor, and then save the file and none of the tags get carried. To defeat image matching just use filters and such (not going into technical details) and that should cover you for the most part while maintain image appearance.
Definitely put a copyright symbol on your pages, which can be done legally without any formal registration. A formal registration can and does provide significant rights though in the US, and is cheap. For information about this see: http://www.ipwatchdog.com/copyright/copyright-application/ and http://www.ipwatchdog.com/copyright/copyright-infringement/ Good luck. -Gene