the problem of transparency

["jiangkening" <jiangkening dhc com cn>]

Dear every professional Linux engineer:
    We are developing a project with gtk of Linux but It is very hard for us
to deal transparency problem.
    The following is  about the detail of the problem.
    We want loading two JPG files, one of both is background picture ,
and the other is transparecy picture over the background  picture . Can you
tell us the method of realizing it with gtk of imlib.
    We are using the following method to deal it but failed

 GdkImlibImage *im;
 GdkImColor color;
 int r ,g ,b;
 gdk_imlib_get_image_shape(im,&color);
 r = color.r
 g = color.g
 b = color.b
 color.r = 255;
 color.g = 255;
 color.b = 255;
gdk_imlib_set_image_shape(im,&color);

the picture of im can not be changed to be transparency.
We feel very upset and please tell us how to slove the problem.

thanks very much