you need to watch what variable you are assigning...
replace:
Code:
{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}
with:
Code:
{if $item.is_pimage eq 'W' }{assign var="imageid" value=$item.variantid}{else}{assign var="imageid" value=$item.productid}{/if}
{include file="product_thumbnail.tpl" productid=$imageid image_x=100 product=$item.product tmbn_url=$item.pimage_url type=$item.is_pimage}