def insert_url(self, url):
        '''download and insert a new item into the cache
        return the information (stamp, hash) on success None otherwise
        item -- a path to an image
        '''
        path = os.path.join(tempfile.gettempdir(), "avatars")
        try:
            urlretrieve(url, path)
        except IOError:
            log.warning("Can't read url avatar")
            return None

        return self.insert(path)