Lynda, this may be a bit old-fashioned but it does work: assuming that white is the image's background color, you can quickly create a msk in Paint by saving a bmp image as "monochrome bitmap" with .msk suffix. Put it in the same folder as the original bmp and include it in your project. The mask will be correctly applied when you use the original bmp on a button.
FWIW I used to open the msk image again and black out all the "inside" of the non-transparent part so that any white pixels in there stay white.
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
