Well, I think you should detail a little bit more your question, e.g. I don't know where did you place the code you mention. Anyway, have you tried to do this:
In the file
app/design/frontend/your-package/your-theme/email/order/items/order/default.phtml
which should be based on
app/design/frontend/base/default/email/order/items/order/default.phtml
you can place this code after <?php $_order = $this->getItem()->getorder(); ?>
<?php $_product = Mage::getModel('catalog/product')
->setStoreId($_item->getOrder()->getStoreId())
->load($_item->getProductId());?>
and then, wherever you want to place your product image you should place
<img src="<?php echo Mage::getModel('catalog/product_media_config')
->getMediaUrl($_product->getThumbnail()); ?>"
alt="<?php echo $_item->getName() ?>" />
Hope this helps you!