Get image previously registered with key rkey.
        If key not exist, raise StockImageException

def get(cls, rkey):
        """Get image previously registered with key rkey.
        If key not exist, raise StockImageException
        """

        if rkey in cls._cached:
            logger.info('Resource %s is in cache.' % rkey)
            return cls._cached[rkey]
        if rkey in cls._stock:
            img = cls._load_image(rkey)
            return img
        else:
            raise StockImageException('StockImage: %s not registered.' % rkey)