If for categories you use a list:
skin1/customer/main/products_list.tpl
Code:
<div class="image"{if $config.Appearance.thumbnail_width gt 0 || $product.tmbn_x gt 0} style="width: {$max_images_width|default:$config.Appearance.thumbnail_width|default:$product.tmbn_x}px;"{/if}>
<a href="{$url}">{include file="product_thumbnail.tpl" productid=$product.productid image_x=$product.tmbn_x image_y=$product.tmbn_y product=$product.product tmbn_url=$product.tmbn_url}</a>
If you use columns:
skin1/customer/main/products_t.tpl
Code:
a href="{$product.alt_url|default:$product.page_url|amp}">{include file="product_thumbnail.tpl" productid=$product.productid image_x=$product.tmbn_x image_y=$product.tmbn_y product=$product.product tmbn_url=$product.tmbn_url}</a>
You need to edit the sizes and such defined by smarty in the product_thumbnail include line. You also need to edit the div class image inline styling defined by smarty if you use the list.