public function getCoverImageGuid() {
		if ($this->getSize() == 0) {
			return 0;
		}

		$guid = $this->cover;
		$imageList = $this->getImageList();
		if (!in_array($guid, $imageList)) {
			// select random photo to be cover
			$index = array_rand($imageList, 1);
			$guid = $imageList[$index];
			$this->cover = $guid;
		}
		return $guid;
	}