Here are two ways to do this...
1) you should create two separate pictures- one for your "up" and one for your "over." Create a blank keyframe in the "up" frame and place the first image in there. Then create a blank keyframe in the "over" frame and put the second image in it.
2) make two layers. in the "up" frame of Layer1, insert a blank keyframe and then insert your image (of the full laptop), and in the last 3 layers, just insert frames (so that the image will show up in all states). In Layer2 (which should be on "top" or above your Layer1), make a blank keyframe in the "up" frame. Make another blank keyframe in the "over" frame and insert an image of the changing screen only (the image will have to use transparency), lining it up to match Layer1.
either way you'll get the same results.