Stochastic resonance (SR) in a bistable system with a binary input signal is studied by numerical simulation. The mutual information (MI) is used as a measure of information. We show that MI shows the resonance peculiar to SR and that the output MI of the bistable system can exceed the input one, that is, MI can have gain. The dependence of the resonance signal and the gain of MI on the noise cutoff frequency is studied and it is shown that the noise with large cutoff frequency is favorable for SR and for the gain of MI. To show the characteristics and the benefits of this type of SR and the gain of MI, the behavior of SR in the bistable system for image data is simulated. The simulation results show that this system can improve the visibility of images.