My includes/languages/english/extra_definitions/MYTEMPLATE/blank_sidebox_defines.php looks like this:

<?php
/**
* blank sidebox definitions - text for inclusion in a new blank sidebox
*
* @package templateSystem
* @copyright 2007 Kuroi Web Design
* @copyright Portions Copyright 2003-2007 Zen Cart Development Team
* @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
* @version $Id: blank_sidebox.php 2007-05-26 kuroi $
*/

define('BOX_HEADING_LOGO2_SIDEBOX', '');
define('TEXT_LOGO2_SIDEBOX', '<a href="http://www.facebook.com/pages/Chief-Okemos-Sportsmans-Club/310761025377"><img src="includes/templates/cosc/images/facebook.gif"></img></a>');

?>

I made my own image for the sidebox.