You can override these meta tags directly in your K2 template overrides.
E.g. in item.php you could do (at the very top of the file):
<?php
/**
* @version 2.9.x
* @package K2
* @author JoomlaWorks https://www.joomlaworks.net
* @copyright Copyright (c) 2006 - 2019 JoomlaWorks Ltd. All rights reserved.
* @license GNU/GPL license: http://www.gnu.org/copyleft/gpl.html
*/
// no direct access
defined('_JEXEC') or die;
$image = 'http(s)://OTHER_SERVER/media/k2/items/src/'.md5("Image".$this->item->id).'.jpg';
$document->setMetaData('og:image', $image);
$document->setMetaData('image', $image); // Generic meta
?>