The default admin email can be found in woocommerce/templates/emails/admin-new-order.php
the order items table is called in line 28, with a function called email_order_items_table. This function also takes an argument for including the thumbnail (see the class-wc-order.php, line 1052). if you copy the email templates to your theme folder, you can edit the admin-new-order.php file and use the following on line 28:
<?php echo $order->email_order_items_table( false, true, false, true ); ?>