Multiple Masking Please Help me

Hi,

I have a code that duplicates a movie clip that then falls across a picture. They are sort of like water drops on a window but I want the picture to only be seen through the water drops. My problem is I can’t get more than one drop to mask the picture at a time. I have tried code in the duplicated MC and I have tried pasting a bunch of instances to the stage and creating a mask later. Please help me out. The following is the code of the duplicated movie clip where _root.drop is the falling mask and _root.bg is the picture I want to see through the drops.

onClipEvent (load) {

var framecount = 0;
if(this != _level0.drop){
this._x = random(795) + 15;
this._y = -30;
}
}
onClipEvent (enterFrame) {
framecount += 1;
if(this != _level0.drop) {this._y += (framecount/36)*50;}
if(this._y > 570) {this.removeMovieClip();}
}

Thanks for your time.

btw, a published edition of what I currently have is available URL=http://noxnerd.tripod.com/droplets.txt]here.