Looking in /customer/main/cart.tpl
I find this section:
{section name=product loop=$products}
{if $products[product].hidden eq ""}
<tr><td class="PListImgBox">
<a href="product.php?productid={$products[product].productid}">{if $products[product].is_pimage eq 'W' }{assign var="imageid" value=$products[product].variantid}{else}{assign var="imageid" value=$products[product].productid}{/if}{include file="product_thumbnail.tpl" productid=$imageid image_x=$config.Appearance.thumbnail_width product=$products[product].product tmbn_url=$products[product].pimage_url type=$products[product].is_pimage}</a>
What do I change to -1?