Not sure if this will help but the process I would use for ImageReady rollovers (I dontuse them but all he same).
Open Image, Use slice tool to create slice. With slice active select the Rollover palette and click on the New State Icon (Similar to the New Layer Icon). Then I would use Control + H (Hide) and then modify the image. Then click the new state icon again to get the next state etc.
Similar to keyframing in a 3D application or Flash.