Anyone know why the picture would shake slightly

Hi,
Since you did not give a lot of detail as to the type of shaking as in from side to side or top to bottem ect.and to model chassis info ect. media source ,scan rate you are using and so on it will be very hard to say but if this is regular and constant problem on all channels and sources basically every thing you watch I would say you are seeing the beginnings of some type of deflection failure in it's very early stages which is a component/connection breaking down and I assure you it will only get worse. Get a service tech/repair shop to check it out asap.
Regards,
Cid