Hmm.. Right now you can't do it without extending the class, because all the queries are album dependent. If no current album is found, they return null. I'm wondering if it is worth adding a function that would return all images regardless of the album.
In any event, for now, you can just subclass ImageGalleryPage.php to something like MyImageGalleryPage and create a controller function like:
function AllImages()
{
return $this->getComponents('GalleryItems');
}
then create /mysite/templates/Includes/AlbumList.ss and do your <% control AllImages %> in there. That's probably the quickest solution.
Post your code if you get stuck.