You probably want to add a
right after the previous line:
Code:
{include file="product_thumbnail.tpl" productid=$product.productid image_x=$product.image_x image_y=$product.image_y product=$product.product}
Other than that, it looks like you've put it in the right spot. I have some other customizations in the product.tpl that may be the difference. What exactly isn't looking right? Want to post a link?