Cambia:
$html .= '<div style="'.$image_width.'px;float:left;padding:3px;p adding-right:6px;height:100%;">
<img src="components/com_virtuemart/shop_image/category/'.find_cat_image($category_tmp[$row_list[$n]]["category_child_id"],$db_username,$db_password,$db_name).'"width='.$wi dth.' height='.$height.'>
</div>';
Por
$html .= '<div style="'.$image_width.'px;float:left;padding:3px;p adding-right:6px;height:100%;">
<a href="'. $sess->url(URL."index.php?page=shop.browse&categor y_ id=".$category_tmp[$row_list[$n]]["category_child_id"]) .'" '.$append.'>
<img src="components/com_virtuemart/shop_image/category/'.find_cat_image($category_tmp[$row_list[$n]]["category_child_id"],$db_username,$db_password,$db_name).'"width='.$wi dth.' height='.$height.'/></a>
</div>';