Use below filter function to add the product thumbnail image below the product description on the template. Add the code to your functions.php file within your theme.

function add_product_image( $description, $item_id, $item ) {
	$product = $item->get_product();

	if ( $product ) {
		$description = sprintf( '<table style="border:none;padding:0;margin:0;"><tr><td width="10%%" style="border:none;padding:0;"><img src="%s" width="50" height="50"/></td><td width="40%%" style="border:none;padding:0;text-align:left;">%s</td></tr></table>', wp_get_attachment_url( $product->get_image_id() ), $description );
	}

	return $description;
}
add_filter( 'wpi_item_description_data', 'add_product_image', 10, 3 );