After effects - image masks

I have some HQ images that will fly in from left then after a second a rectangular mask will drop in from above
the Masked bit of the image will move to the left then as its moving - the next image flys in from the left and after a second that gets a mask from above and then that moves to the left I need it with 5 images so the end result is five masked images. I'd like them to move nicely and freely and the mask to look good when it drops down.