you have to put your code in products.tpl for it to appear in the products list and product.tpl for it to appear on product details
the code you ould use in products.tpl would be along the lines of
Code:
{if $products[product].param00 ne ""}
[img]{$ImagesDir}/3for2.gif[/img]
{/if}
and for product.tpl
Code:
{if $product.param00 ne ""}
[img]{$ImagesDir}/3for2.gif[/img]
{/if}
the code basically says ...
if param00 does not equal nothing / empty do something
replace param00 with the correct extra field that you are using. Use phpmyadmin to check or make a guess
add this code below the call for the thumbnail