[balinor]

Edit cart.tpl and either delete this whole part:

<TD width="90" align="center" valign="top">
{if $active_modules.Special_Offers ne "" and $products[product].have_offers}
{include file="modules/Special_Offers/customer/product_offer_thumb.tpl" product=$products[product]}
{else}
{include file="product_thumbnail.tpl" productid=$products[product].productid image_x=$products[product].image_x image_y=$products[product].image_y product=$products[product].product tmbn_url=$products[product].tmbn_url}
{/if}
</TD>

or just change the image_x=$products[product].image_x to image_x=150 and delete the image_y part.