How can I choose only the round button from square button space?

[bkna besta cc]

Hi,
I would put pixmaped buttons, instead of labeled buttons.
Thus, I have made round-shaped button XPM files.
As I know I can not remove the edge space.
I used the function, gdk_pixmap_colormap_create_from_xpm_d ().
However,
even I make them transparent, buttons have the unnecessary edges.
I would display only the round button icons.

Is it possible and if so, how?

Thank you,
Bokyun Na