I've got it to show up on the web site, but not on the emails.
This is what I got right now:
<a href="{$url}">{include file="product_thumbnail.tpl" productid=$product.productid image_x=50 image_y=50 product=$product.product tmbn_url=$product.tmbn_url}</a>
I don't know how to do it with the image.php