Selective image encryption has been considered as an effective method to improve the encryption speed and computation resource by reducing considerable amount of data. In the present paper, the weaknesses of recent method of selective image encryption are presented analytically and the method of security improvement is proposed. The security for selective image encryption is improved in a way that number of selectively significant bits for encryption vary from pixel to pixel. This leads to be uncertain data for encryption and decryption. The examples are to demonstrate and verify for security weaknesses of the recent method and the effectiveness of the proposed method.