Hi Laurie,
There are surely several ways to do this, but one way might be to:
1) Add the first image to a new comp.
2) Add a solid layer on top. Comp size, colour doesn’t matter.
3) Add the TV screens on top. These might be 15 individual images, duplicated (you could duplicate these using one of Eran’s duplication methods outlined in his recent AE podcasts here at the Cow), or one image of all 15 TVs. Crucially, the screens will have to be transparent or masked out to allow the image below to show through.
4) Select the solid layer and draw a rectangular mask around the 1st TV screen. Duplicate the mask so there’s one for each screen.
5) Set the track matte of the first image to alpha matte. The transparent parts of the solid will now be used to make areas of the image transparent.
6) Put the 2nd image on the bottom.
7) Now when you animate the opacity of the masks, the 2nd image should show through.
You might also want to check out Aharon’s jumbotron / TV style AE podcasts.
This is admittedly quite a time-consuming method, so maybe someone else will have some better ideas.
Simon Bonner
youtube.com/simonsaysFX