If there is no WWW. don’t worry it doesn’t matter. Just make sure that the http:// does not appear twice at the beginning and use the direct link tag. Just tried with an image shack image on the critique page and it was all systems GO!
As you know, you can store image files in a private gallery here on the CPS site. Or you can use a storage site like ImageShack or Flickr. At the end of the guide, there’s information about how to get the URL for your image file from each of these different storage locations.