I have checked my file
tpl_modules_main_product_image.php
and if you have not changed anything, line 19 should read
Code:
  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>';
all on one line.

I hope this helps?