Magento 1: Add the product image on the email order (template/email/order/items/order/default.phtml)

add-product-image-email-order.php

<?php $_item = $this->getItem() ?>
<?php $_order = $this->getItem()->getOrder() ?>

<?php if ($_item->getProductType() == Mage_Catalog_Model_Product_Type::TYPE_CONFIGURABLE) {
    $parentId = Mage::getModel('catalog/product_type_configurable')
                ->getParentIdsByChild($_item->getProductId());
    $_product = Mage::getModel('catalog/product')  
                ->setStoreId($_item->getOrder()->getStoreId())  
                ->load($parentId);
} else {
    $_product = Mage::getModel('catalog/product')  
                ->setStoreId($_item->getOrder()->getStoreId())  
                ->load($_item->getProductId());  
} ?>
...
...
<td class="cell-content product-image" style="padding: 3px 9px; vertical-align: top;">
  <img src="<?php echo $this->helper('catalog/image')->init($_product, 'thumbnail')->resize(125,125); ?>"  width="125" height="125" alt="<?php echo $_item->getName() ?>" style="border: 1px solid #ddd;" />
</td>