[plt-scheme] Loading transparent png

Hi all,

I'm loading a transparent PNG to a canvas with:
(instantiate canvas% ()
                         (parent hpane-top)
                         (style '(transparent))
                         (paint-callback (lambda (canvas dc)
                                           (send dc draw-bitmap
cross-bitmap 0 0)))))

However, the PNG is not showing up as transparent but with a white
background, or probably it's shown as transparent but canvas is
'drawing' a white background which I can't seem to remove.

What's the best way to do this?

Thanks,

-- 
Paulo Jorge Matos - pocm at sat inesc-id pt
Web: http://sat.inesc-id.pt/~pocm
Computer and Software Engineering
INESC-ID - SAT Group