Alright. Everything I wrote previously was correct... but moot.

Now how about a solution, huh? (for those having a problem.)

Replace /includes/templates/YOUR-TEMPLATE/templates/tpl_modules_main_product_image.php with the following:
Code:
<?php
/**
 * Module Template
 *
 * @package templateSystem
 * @copyright Copyright 2003-2005 Zen Cart Development Team
 * @copyright Portions Copyright 2003 osCommerce
 * @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
 * @version $Id: tpl_modules_main_product_image.php 3208 2006-03-19 16:48:57Z birdbrain $
 */
?>
<?php require(DIR_WS_MODULES . zen_get_module_directory(FILENAME_MAIN_PRODUCT_IMAGE)); ?>

<div id="productMainImage" class="centeredContent back">
  <!-- begin zen lightbox v1.3 aclarke 2006-10-20 -->
  <script language="javascript" type="text/javascript"><!--
document.write('<?php echo '<a href="' . zen_image_lightbox_IH2_url($products_image_large, addslashes($products_name), '800', '600') . '" rel="lightbox[gallery]" title="' . addslashes($products_name) . '">' . zen_image($products_image_medium, addslashes($products_name), MEDIUM_IMAGE_WIDTH, MEDIUM_IMAGE_HEIGHT) . '<br /><span class="imgLink">' . TEXT_CLICK_TO_ENLARGE . '</span></a>'; ?>');
//--></script>
  <!-- end zen lightbox v1.3 aclarke 2006-10-20 -->
  <noscript>
  <?php
  echo '<a href="' . zen_href_link(FILENAME_POPUP_IMAGE, 'pID=' . $_GET['products_id']) . '" target="_blank">' . zen_image($products_image_medium, $products_name, MEDIUM_IMAGE_WIDTH, MEDIUM_IMAGE_HEIGHT) . '<br /><span class="imgLink">' . TEXT_CLICK_TO_ENLARGE . '</span></a>';
?>
  </noscript> 
</div>
Basically, the problem was with LARGE_IMAGE_WIDTH and LARGE_IMAGE_HEIGHT so I hard coded them with '800' and '600'.

Bada Bing. Not an elegant solution... but it works!