OK,well that makes sense, it was just a suggestion of an easy work around.

You are going to have to try and merge the code from additional_images.php (this creates an array of the additional images which is used by tpl_columnar_display) with the ' case 'PRODUCT_LIST_IMAGE' section of product_listing.php (which also produces an array that is then used by tpl_columnar_display).

Or thats how I would start to think about it...

You can probably simplify the code quite a bit if you know that there is going to be only a single additional image