public function getPreviousImage($guid) {
		$imageList = $this->getImageList();
		$key = array_search($guid, $imageList);
		if ($key === false) {
			return null;
		}
		$key--;
		if ($key < 0) {
			return get_entity(end($imageList));
		}
		return get_entity($imageList[$key]);
	}