masking 2 pictures problem

Hello

We are going to film in a food store and going to film the shelves in profile on a dolly.

So when the camera is going to pass shelf number two I want another picture to be "masked" in. That picture is starting in white (white wall) and where the wall ends, a man stands. So the camera is always going from left to right at the same speed, just trhough different "rooms".

Do you know any good tutorial on masking (videotutorial preferable).

Thanks in advance!
Erik/sweden