look in includes/templates/YOUR_TEMPLATE/sideboxes/tpl_logo_sidebox.php

I've bolded the parts that apply to your situation.
Code:
<?php
//
// Blank Sidebox Mod
// includes/templates/templates_default/sideboxes/tpl_blank_sidebox.php
//
//  --------------------------------------------------
//  http://www.MommaMuse.com mod Zen-Cart - by Judi Cox                  
//  --------------------------------------------------
//  zen-cart Open Source E-commerce                                      
//  Copyright (c) 2003-2006 The zen-cart developers                           
//  http://www.zen-cart.com/index.php                                    
//  Portions Copyright (c) 2003 osCommerce                               
//  --------------------------------------------------
//  This source file is subject to version 2.0 of the GPL license,       
//  that is bundled with this package in the file LICENSE, and is        
//  available through the world-wide-web at the following url:           
//  http://www.zen-cart.com/license/2_0.txt.                             
//  If you did not receive a copy of the zen-cart license and are unable 
//  to obtain it through the world-wide-web, please send a note to       
//  [email protected] so we can mail you a copy immediately.
//  --------------------------------------------------
//
// $Id: tpl_blank_sidebox.php,v 1.0 6/24/2006
// Original modification by Carter Harris [email protected] based on the tpl_featured.php file 
// Additionally modified by Judi Cox http://www.mommamuse.com
//
// Link Option - uncomment this if you intend to use the logo as a link
//$logoimage = '<a href="' . LOGO_SIDEBOX_LINK . '">' . zen_image($template->get_template_dir(LOGO_SIDEBOX_IMAGE, DIR_WS_TEMPLATE, $current_page_base,'images'). '/' . LOGO_SIDEBOX_IMAGE, LOGO_SIDEBOX_IMAGE_TEXT) . '</a>';

//comment this out if you use the above link option.
$logoimage = zen_image($template->get_template_dir(LOGO_SIDEBOX_IMAGE, DIR_WS_TEMPLATE, $current_page_base,'images'). '/' . LOGO_SIDEBOX_IMAGE, LOGO_SIDEBOX_IMAGE_TEXT); 

  $content = '';
    $content .= '<div id="' . str_replace('_', '-', $box_id . 'Content') . '">';

  $content .=  $logoimage;

  $content .= '</div>';
?>