Puzzle solved!
When you look at any object, even if you constantly gaze at it, and you think you are looking at it steadily, but in fact there are slight movement of the eye taking place, not under your control.. very slight and relatively fast movement. So when the eye moves a few micrometers, and due to the optic nerve response delay, the old image is still there and needs some time (very short time) to adapt to the new image (location). That's why you see the image is shaking (or moving) as you said. If your optic nerve is healthy and strong (nothing personal please!) then it can refresh the image fast enough not to notice any (motion) caused by optic nerve response delay.