Scrolling image button

i would <3 anyone forever if they could point me in the right direction with this…

i want to create a square button that when moused over it scrolls a movie…(masking)

here is a good example:

visit and go to her “laboratory work” section, and it’s the little square things at the top…they aren’t easy to miss

i want something like that but i’m not really sure how to execute it…please help…i will be your slave forever