Sprite mask from PNG

[Davism72]

I have a PNG of an irregularly shaped picture frame. The center of the frame, where the picture would go, is completely transparent. I'd like to use this image to create a graphics object in the shape of the PNG's transparent section which I can use as a sprite mask. Any tips on how to accomplish this?

I've looked through the examples for sprite masks and bitmap alpha masks. I feel like I need something between the two. Is there some way to create the correct shape using a BitmapData object, then use that as a sprite mask? I couldn't find a way -- seems like the mask needs to be a graphics object.

Thanks in advance.