Display product thumbnail in Woocommerce email notifications

by

You’d better use woocommerce_email_order_items_args filter.
It can be done this way:

function add_product_thumbnail_to_wc_emails( $args ) {
    $args['show_image'] = true;
    $args['image_size'] = array( 100, 100 );
    return $args;
}
add_filter( 'woocommerce_email_order_items_args', 'add_product_thumbnail_to_wc_emails' );

The code has to be placed to functions.php