After some time (it depends on the frequency being used), and with people’s attention being almost totally fixated on the image, people experience a sensory resonance frequency that can be created by several methods. Among them are:
creating a video signal for displaying an image on the monitor, the image having an intensity;
modulating the video signal for pulsing the image intensity with a frequency in the range 0.1 Hz to 15 Hz; and
setting the pulse frequency to the resonance frequency.
Probably already been done, but it could be attacked from many different angles.