you would need to edit the image recordsets on the admin_gallery/manage_images.php page, and on the index pages for each subfolder in the galleriies/ folder.
you would need to change the order by clause:
ORDER BY display_order ASC
ORDER BY display_order DESC
once you make this change the gallery order will be reversed, so you will need to re order any existing galleries, but any new images you add will automatically show at the beginning.