Add Additional images on product listing page

I originally posted this question in the general customization section, however perhaps this is a better area for this question.

I am building a clothing store for a client. Many of the items have an additional image of the back of a shirt for instance. I am trying to figure out how to show the additional image next to the right of the product main image on the product listing page (not the product info page). I have tried modifying code on tpl_modules_product_listing.php, tpl_modules_main_product_image.php, additional_images.php, tpl_modules_additional_images.php, and product_listing.php and I am having no luck on locating the code where I can make this happen.

Can anyone tell me how to accomplish this? It seems like it should be a simple copy and paste of this code...

require($template->get_template_dir('/tpl_modules_additional_images.php',DIR_WS_TEMPLATE, $current_page_base,'templates'). '/tpl_modules_additional_images.php'); ?>  

...if I can only just find where to paste it.

Your help is greatly appreciated.

Original Thread http://www.zen-cart.com/forum/showthread.php?p=752491&posted=1#post752491

bump

I am still desperately looking for a way to get additional product image to appear next to the main product image on the category product listing page. Does anyone have an idea on how to do this?