1.Create a new movie.
2.Import two images into flash.
3.Convert the images to movieclips.
4.Give both the movieclips instance names, I gave "angel" and "horse".
5.Now create the masking object, in this case it is a small circular object.
6.Convert the circular object into a movie clip give it an instance name of "masker".
7.Create three buttons
8.Before I give actionscript to the buttons I give them Instance names. I gave "left", remove" and "right".
9.Select the first frame of the current layer, press f9 to open the actions window and enter the folowing actions:

right.onPress = function() {
angel.setMask(null);
horse.setMask(masker);
};

left.onPress = function() {
horse.setMask(null);
angel.setMask(masker);
};

remove.onPress = function() {
horse.setMask(null);
angel.setMask(null);
};

masker.onEnterFrame = function() {
this._x = _xmouse;
this._y = _ymouse;
};

10.Test the movie.