Ok, try this:
in details.php find:
show_image($image_row, $mode, 0, 1);
Insert above:
$site_template->register_vars("thumb", get_thumbnail_code($image_row['image_media_file'], $image_row['image_thumb_file'], $image_row['image_id'], $image_row['cat_id'], $image_row['image_name'], $mode, 0));
Then in your zip.html media template use {thumb} tag