Random movement that stops- mask

Not sure if this is the best place to post this thread, but…

I’m looking for a way to have a mask randomly reveal a photo I have… by using square shapes (or any random shapes, i guess)… but like… the shapes do random moving things as they reveal parts of the photo.

Does that make any sense? I know it’s do-able… I just don’t know how to do it!! I’m trying to look for pretty cool special effects, but I’m not succeeding.