Users Interaction To Delete Mask?

Hi guys,

I have two images, one of which lays on top of the other. What I’m trying to achieve is that the user watching the flash can interact by clicking down on their mouse and wipe away the top layer to reveal the bottom image.

Obviously this is done through masks, but I don’t know how to do it so the user can do it, and it will only wipe away the bits they go over.

Help with this would be HUGELY appreciated!

Many thanks,