Well, if you had product images all you would probably need to do is replace
in products_t.tpl ...
<a href="product.php?productid={$products[product].productid}&cat={$cat}&page={$navigation_p age}">{include file="product_thumbnail.tpl" productid=$products[product].productid image_x=$products[product].tmbn_x|default:$config.Appearance.thumbnail_width image_y=$products[product].tmbn_y product=$products[product].product tmbn_url=$products[product].tmbn_url}</a>
or products.tpl (depending on which you use)...
<a href="product.php?productid={$products[product].productid}&cat={$cat}&page={$navigation_p age}{if $featured eq 'Y'}&featured{/if}">{include file="product_thumbnail.tpl" productid=$products[product].productid image_x=$config.Appearance.thumbnail_width product=$products[product].product tmbn_url=$products[product].tmbn_url}</a>
with....
{include file="product_thumbnail.tpl" productid=$product.productid image_x=$product.image_x image_y=$product.image_y product=$product.product tmbn_url=$product.tmbn_url id="product_thumbnail" type="P"}
(please note: I do not know if this will work as I have not tried it)
I'm still wondering why you would want a full size image on your category pages.