This is the effect of physiological nystagmus. If you gaze at a black dot, trying not to avert eyes from it, then after 30 seconds black and white parts of the image will begin vibrating. If after that you shift your gaze to the white dot, you will see a set of white squares on the black background (i.e. consistent image), imposed against the genuine picture. This consistent image will always shift along the picture, no matter how you try holding it in place.