Hello,
The image is not stored in the db.
It is renamed using an md5() functions and it is being pulled using a file exists check.
(components/com_k2/models/item.php)
if (JFile::exists(JPATH_SITE.DS.'media'.DS.'k2'.DS.'items'.DS.'cache'.DS.md5("Image".$item->id).'_XS.jpg'))