A Question for My Online Portfolio

Hi guys.
I’m in dire need of help again.
The thing is, I’m doing this small litte temporary portfolio in Flash and I’m terrible att ActionScript.
I load external SWFs (which are pictures) on to the main SWF from a menu. Now, I want have sort of a beer coaster on these pictures to appear. I’ve put a simple bitmap to have under which I’m pretty pleased with.
But the problem is that I don’t want this coaster to show before I’ve pressed on any button. When I press a button in the menu I want the beer coaster to be transitioned in using a mask layer.
This is how the thing looks right now: http://www.subfusc.se/temporary/temporary.html
Do you know what I’m trying to explain?

Any help is thoroughly appreciated!

Thank you so much,
Mattias (Subfusc)