[cflsystems]

It's this line

  <a href="popup_image.php?type=D&amp;id={$product.productid}" onclick="javascript: setTimeout(function() {ldelim}imagesPreviewShow('dpi', dpimages, dpioptions);{rdelim}, 200); return false;" target="_blank">{$lng.lbl_view_detailed_images|substitute:"counter":$images_counter}</a>

in this file
skin/common_files/modules/Detailed_Product_Images/popup_image.tpl