Bitmap -> Movieclip Hit Area Question


I’m using actionscript to place a bitmap into an empty movieclip like so:

var bitmap_mc:MovieClip = new MovieClip;

addChild( bitmap_mc );

    bitmap_mc.addChild( bitmapPNG );

bitmapPNG is an irregular shaped png of a person, but when I use the movieclip as a button, the hit area is the square shape of the movieclip, not the silo. How do I make the irregular shape of the silo the hit area, and not the surrounding square of the moviclip?