Hi. All i can do is give you the code which can give you back the avatar of a K2 user:
<?php
$userId = 100; // This should be the id of the user. 100 is just an example.
$user = JFactory::getUser($userId);
require_once JPATH_SITE.'/components/com_k2/helpers/utilities.php';
$avatar = K2HelperUtilities::getAvatar($user->id, $user->email, 50);
?>
However i cannot know what are the parts in Kunena you need to put this.
