First off, all images have official copyrights on them, it is just that a registered copyright provides for much more substantial damages, in infringement cases.
You can register them online and it is actually cheaper and easier, as well, online tracking is available for those who use that service. You can also register published works but they have to be filed separate from those that are not published yet.
Here is a link from the Library of Congress with FAQs http://www.copyright.gov/eco/faq.html
Follow the menu as it takes you to the point where you can actually start uploading and filling for copyright registration. Hope this helps.
The 90 day rule is only in effect if infringement takes place within 90 days of first publication. Providing a 90 grace period for the owner of the image.