[plt-scheme] bitmaps, buttons, transparency

I've got some xpm bitmaps that I'm using as the label for some
buttons. When I view these bitmaps in image viewers, they appear to
have transparent backgrounds. When I stick them in a button, those
areas appear black.

Does anyone have some example code they can share for how to get the
button background to be the same as (get-panel-background) ?

Ed