Slide In and mask

Hi Guys,

I need a help from you to explain how this effect at Masmadera can be achieved. I am talking about the transition effect when each new image/swf being upload.

I have incorporate the slide in/out with easing prototype I got from the web to my project successfully, but I am stuck with the masking effect. Here is the prototype:

[AS]Math.easeOutCirc = function(t,b,c,d){
t/=d;
t–;
return cMath.sqrt(1-tt)+b;
}

MovieClip.prototype.slideTo = function(newVar, oldVar, prop){
var t = 0;
var d = 30;
var delta = newVar - oldVar;

var oldv = oldVar;
var newv = delta;

this.onEnterFrame = function(){
	t++;
	if (t<=d){		
		this[prop] = Math.easeOutCirc(t,oldv,newv,d);	
	} else {
		delete this.onEnterFrame;
	}	
}

}[/AS]

I appreciate if anyone can help me especially if you can post the fla.

I hope this is not too much to ask.

Cheers,
Beebs