Xml image masking

1 
myPhoto = new XML();
myPhoto.ignoreWhite = true;
//this.attachMovie("round_main", "round_main", 1, {_x:77.5, _y:90});
myPhoto.onLoad = function(success) {
    numimages = this.firstChild.childNodes.length; 
    spacing = 50;
    for (i=0; i<numimages; i++) {
        this.picHolder = this.firstChild.childNodes*;
        this.thumbHolder = thumbnails.createEmptyMovieClip("thumbnail"+i, i);
        this.thumbHolder._x = i*spacing;
        this.thumbLoader = this.thumbHolder.createEmptyMovieClip("thumbnail_image", 2); 
        this.thumbLoader.loadMovie(this.picHolder.attributes.thmb);
        this.thumbHolder.main = this.picHolder.attributes.main;
        this.thumbHolder.onRelease = function() {
            loader_mc.loadMovie(this.main);
        }
    }
}
myPhoto.load("photo.xml");

/* masking thumbnails */
mask2._x = thumbnails._x; // + thumbHolder._width/2;
mask2._y = thumbnails._y; // + thumbHolder._height/2;
thumbnails.setMask(mask2);

/* masking main image */
mask3._x = loader_mc._x;
mask3._y = loader_mc._y;
loader_mc.setMask(mask3);

I would like to mask main image when is loaded from xml, I’m trying this for 10 day allready and I couldn’t do it, the thing is that thumbnails masking is working fine but main image not.

thanks