Help with swapping Movie Clips

I am currently working on a project of a commercial flash box thing for their website.
I have a thing where I need a bunch of images to wipe in this certain way. They are in mc’s, and just sitting there, while I have squares moving on top of them doing the wiping. I need to know how to swap these image mc’s out when it hits a certain frame e.g. frame 70.

Any help would be greatly appreciated.