Show thumbnail images in WooCommerce order emails

Thumbnail images can be shown in WooCommerce order emails with the filter woocommerce_email_order_items_args

add_filter( 'woocommerce_email_order_items_args', 'bt_email_order_items_args', 10, 1 );
/**
 * WooCommerce
 * Show thumbs in Order Email sent to customers
 */
function bt_email_order_items_args( $args ) {

    $args['show_image'] = true;
    $args['image_size'] = array( 150, 150 );

    return $args;

}

If you want the images to be larger and there is a valid image size set adjust the image size array.