I am trying to animate an image of a smartphone in the middle of the screen so that it looks like pictures on the iphone screen are sliding into place on top of the previous pictures. I am having a tough time creating a mask that shows all of the other elements on the stage(lots of them) but hides only the image that is waiting to slide into view. I remember seeing a tutorial where the person created a shape layer and somehow used it to mask the layer below to do something similar, but I can't seem to find it anymore. Any suggestions are appreciated.
Use a shape -- I personally prefer a solid -- in the precise dimensions of the screen as a track matte. You'll need one track matte per picture you animate.