[plt-scheme] Simple question about bitmap and transparency

hi,
I'm student and I have a simple question, I searched but I didn't find
recent answer.
So :
(define gg (make-object bitmap% "toto.png" 'png #f))

My image is a round logo with transparency. How to have transparency when I
draw it?

(Actually I have a white or black rectangle around my logo, instead of
transparency)

I try with gif, with 'png/mask 'gif/mask and this (send a-bitmap
get-loaded-mask) no way.

Thanks.
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