Use this code to get the actual thumbnail and link to the full sized images.
$newurl = get_bloginfo("wpurl") . "/" . $image->path . "/thumbs/thumbs_" . $image->filename;
$output .= "<a class=\"shutterset_set_1\" rel=\"%GALLERY_NAME%\" href=\"" . $image->imageURL . "\"><img src=\"" . $newurl . "\" " . $image_description . $width_d . $height_d . " border=\"0\" /></a>";
This is a mash-up of “whatleyc”‘s and the above code.