you need to watch what variable you are assigning...
replace:
Code:
{include file="product_thumbnail.tpl" productid=$minicart_contents image_x=$config.Appearance.thumbnail_width product=$products[product].product tmbn_url=$products[product].pimage_url type=$products[product].is_pimage}
with:
Code:
{include file="product_thumbnail.tpl" productid=$item.productid image_x=$100 product=$item.product tmbn_url=$item.pimage_url type=$item.is_pimage}