Non rectangle buttons from PNG w/ transparency?

I’d like to be able to create dynamic masks from PNG images by removing/ignoring the transparent pixels so that only the non-transparent parts of the image acts as the mask. A native shape can be any shape and used as a mask, so is there a way to get to that point from a PNG file?

I looked at some bitmapData stuff and you can isolate pixels and change their color, etc., so can you exclude them from the mask, or is there a way to convert (i.e. break apart) an image a runtime?

Any help/thoughts would be most appreciated. TIA!