Woocommerce 3.1 Add product image to order confirmation email not working

Try this code in functions.php inside your theme folder.

add_filter( 'woocommerce_email_order_items_args', 'iconic_email_order_items_args', 10, 1 );

function iconic_email_order_items_args( $args ) {

    $args['show_image'] = true;

    return $args;

}