If in your product.tpl you want to show the thumbnail when there is not detailed images, use:
Code:
{if $active_modules.Detailed_Product_Images ne "" && $images ne ""}
{include file="modules/Detailed_Product_Images/product_images_one.tpl" }{else}{include file="product_thumbnail.tpl" productid=$product.productid image_x=$product.image_x image_y=$product.image_y product=$product.product tmbn_url=$product.tmbn_url id="product_thumbnail"}{/if}