Thank you for reply, I just removed it.
I added style="text-decoration:none;" in tag <a> in the line
Code:
$script_link = '<script language="javascript" type="text/javascript"><!--' . "\n" . 'document.write(\'' . ($flag_display_large ? '<a style="text-decoration:none;" href="' . zen_lightbox($products_image_large, addslashes($products_name), LARGE_IMAGE_WIDTH, LARGE_IMAGE_HEIGHT) . '" rel="' . $rel . '" title="' . addslashes($products_name) . '">' . $thumb_slashes . '<br />' . TEXT_CLICK_TO_ENLARGE . '</a>' : $thumb_slashes) . '\');' . "\n" . '//--></script>';
includes/modules/MY_TEMPLATE/additional_images.php