gdk_draw_rgb_32_image() always display black background?

[yanxin <ycui wmsvision com>]

I am using gdk_draw_rgb_32_image() to display a transparent pixbuf into
a drawable, here is my function call:
        gdk_draw_rgb_32_image(
   psWidget->window,
   psWidget->style->bg_gc[psWidget->state],
   sIntersection.x, sIntersection.y,
   sIntersection.width, sIntersection.height, GDK_RGB_DITHER_MAX,
   gdk_pixbuf_get_pixels( psPixbuf )
    + ( sIntersection.y * gdk_pixbuf_get_rowstride( psPixbuf ) )
    + ( sIntersection.x * gdk_pixbuf_get_n_channels( psPixbuf ) ),
    gdk_pixbuf_get_rowstride( psPixbuf ) );

But I am always getting a black background, how can I display the image
with a transparent background?

Thanks in advance!

Yanxin