Force to show free shipping icon w/ freeoptions on

I use multiple shipping methods on product page. I opened the freeoptions instead of freeshipper(no intention to turn it on.)

On the product information page, I was able to add the below code in tpl_product_info_display.php to show the "FREE SHIPPING ICON" , no matter this product is "always free shipping" or not.

PHP Code:

---

  <!--bof free ship icon  -->
             <li>This item is: <img border="0" class="g_t_m" src="includes/templates/<?php echo $template_dir; ?>/images/free_shipping_pro.png"/></li>

        <!--eof free ship icon  -->

---

But on the product listing page, the "FREE SHIPPING ICON" doesn't show.

In products listing page on each category, which PHP file I can edit to add a free shipping image and edit the section in the below image?

Attachment 11623