I just did some quick research on copyright law through the US Copyright Office of the Library of Congress. If published in the US after Jan 1, 1978, your work is automatically covered by US Copyright Law. You do not need to register your work with the Copyright Office, although it is recommended. If published after march 1, 1989, you do not even need to make a copyright statement, but that too is still recommended.
If your work is registered with the US Copyright Office prior to an infringement of your copyright, you can recoup legal fees and other certain damages. Although there are some restrictions, you can register a group of photographs all at the same time, thereby reducing considerably your level of effort and costs. You need not send hard copies and it appears that the Copyright Office prefers images on CD when registering a group of them. You can get more information on this at
http://www.copyright.gov/. Be sure to go to "forms" and download Form GR/PPh/CON for information/application for group registration of photographs.
Regarding a copyright date, you should use the date that the photograph was published (not when it was taken). The copyright officer that I spoke to would not commit as to whether putting pictures up on PBase represented a form of "publication", but you can read their definition and determine for yourself. It certainly appears to me that posting pics on PBase is a form of publication according to the Copyright Office definition.
Hope this helps,
Garrie Rouse