PHOTO under mask moves w/mouse?

I want to be able to load a jpeg into an MC and make the image in the MC slide around under a mask as you move the mouse.

There’s an example of what I’m talking about on this website:

If you click on any one of the thumbs …then on the left it says “view detail on” …click it and you’ll see what I mean.

If anyone could tell me how (or where I can find out how in a tutorial or something) it’d be great.