A hole in a picture to display another picture – no Flash

I don’t really have an idea how to call this, but I have an example (in Flash) of what I want to be realized in HTML5/CSS/JavaScript (jQuery), and that should make it quite clear:

A hole to the past

Why it is not like this example: The hole should follow the mouse (without clicking), and it should be possible to change the foreground/background images by a “control” (a clickable area).

Can anyone give me some hints how to do that?