Containing a moving image

Sorry if this has been posted or is in the tutorials, I tried to find it, but could not. I understand how to make an object follow the movement of the mouse/stay with the mouse, what I am not sure of iss how to keep the image contained within a frame and still accomplish that goal:
ie - eyeballs that move to follow the mouse cursor, but that also stay within oval of the eyes themselves. Does that make enough sense?