Ok, yesterday I found some clues, but i must use a xpm file (Re: Background image for a window) ...

my $rc_style = Gtk2::RcStyle->new;
$rc_style->bg_pixmap_name ('normal', 'keyboard.xpm');
$window->modify_style ($rc_style);
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i will abandon the attempt to explore the GTk2::Gdk::Window ... :'( ...

turo