In Canada you must register each photo individually - at the cost per photo you won't be able to afford registering each one unless you make a few hundred thousand a year on your work. In the US you can register a number of photos as a "collection" on CD for one price, so that is more cost effective for Americans.
It isn't necessary to register photos (or any works) with the copyright office to hold copyright to the images - it becomess necessary if you find people stealing your work and claiming your copyright - and if you want to sue and claim damages. You can claim damages on unregistered images, but the amount is limited to a certain time frame (I'd have to look it up again) if the image isn't pre-registered. If you registered the image at the time of it's creation, you could conceivably get much more for it.
In order to protect your images you don't really even have to stick the copyright notice on them, though most of us do. Stock sites will not usually accept images with an embedded copyright (such as one that digimark might embed) or with visible copyright notices.
You can attach a non-visible copyright in your IPTC data - when you are adding keywords there is generally a field for copyright notice.
If you are interested in Canadian photography information you can visit Canphoto at
http://www.canphoto.net/ and check the "discussions" section. You must register to view certain sections of the forums (notably the business info) and in order to get approval for those sections you must be a working photographer (whether a business owner or freelance), a registered student photographer)...I forget the other stipulation but it will tell you on the forums. I don't post much there, but I've found the site extremely helpful and support them financially.
Some links:
US Copyright Info:
http://www.copyright.gov/
http://www.naturescapes.net/042005/cw0405.htm
http://www.faqs.org/faqs/law/copyright/faq/part1/
http://www.htmlgoodies.com/beyond/re...le.php/3472671
http://www.chillingeffects.org/copyright/faq.cgi
http://www.copyright.gov/wipo/
CANADIAN Copyright Info
http://www.cla.ca/resources/copyright.htm
http://laws.justice.gc.ca/en/C-42/
http://www.cb-cda.gc.ca/info/act-e.html
http://www.trytel.com/~pbkerr/copyright.html
http://www.edu.gov.mb.ca/ks4/iru/copyright/
http://www.media-awareness.ca/englis...pyright_ov.cfm
http://www.accesscopyright.ca/