Masking loaded movies/images

programming flash
1

I’m using the action script below to load a series of images into my Flash file. I’d like to have a mask above this file that obscures parts of the images. How would I go about masking these loaded movies/images?

createEmptyMovieClip(“pic_holder”, 0);
pic_holder._x = 7;
pic_holder._y = 7;
pic_holder.loadMovie(“house1.jpg”);