try this:
in includes/functions.php find:
$site_template->register_vars(array(
"image_id" => $image_row['image_id'],
Insert above: if (check_permission("auth_viewimage", $image_row['cat_id']) && check_permission("auth_viewcat", $image_row['cat_id']))
{
$image_url = $site_sess->url(ROOT_PATH."details.php?".URL_IMAGE_ID."=".$image_row['image_id'].((!empty($mode)) ? "&mode=".$mode : ""));
}
else
{
$image_url = "";
}
$site_template->register_vars("image_details_url", $image_url);
Then you can use {image_details_url} in thumbnail_bit.html template.