It sounds like you would have to make a custom gallery that instead of getting a list of all images for 1 gallery, it gets 1 image for all galleries. Each gallery is powered by XML that is generated from the mediarss.php file, so you have to make a new php file (copy mediarss.php as a starting point), then adjust the database query to suit your needs. Then you will have to modify your gallery to use this new php file instead of the mediarss.php to power the gallery.
-justin