Masking bitmaps

Hi, I was reading some threads to see if there was an answer specific to this but can’t seem to find one so sorry if this is a repeat question.

I am trying to mask images under a bitmap (it’s a shape but complex, say something like a star and is drawn in photoshop/fireworks and exported into flash) - I was hoping to be able to use the outline of the image to mask the image under it but can’t seem to figure out how to use only the outline of the image, not the square that is the size of the bitmap.

Anyone know how I can do this?