Problem with TGraphic control and Trasparent.

Hi,
I have a problem in my Tgraphic Control, if i compile this code:
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MainSkin:           TPicture;
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procedure Tsk.SetSkinPic(Value: TPicture);
begin
  MainSkin.Assign(Value);
  MainSkin.Bitmap.Transparent:= true;
  MainSkin.Bitmap.TransparentMode:= tmAuto;
  MainSkin.Bitmap.TransparentColor:= clBlack;
  //Canvas.CopyRect(rect(0,0,self.Width,self.Height),
  //                MainSkin.Bitmap.Canvas,
  //
rect(GetOffPicLeft,GetOffPicTop,GetOffPicRight,GetOffPicBottom));
  Canvas.Draw(0,0, MainSkin.Bitmap);
end;

The black area of the bmp is trasparent but:

procedure Tsk.SetSkinPic(Value: TPicture);
begin
  MainSkin.Assign(Value);
  MainSkin.Bitmap.Transparent:= true;
  MainSkin.Bitmap.TransparentMode:= tmAuto;
  MainSkin.Bitmap.TransparentColor:= clBlack;
  Canvas.CopyRect(rect(0,0,self.Width,self.Height),
                  MainSkin.Bitmap.Canvas,

rect(GetOffPicLeft,GetOffPicTop,GetOffPicRight,GetOffPicBottom));
  //Canvas.Draw(0,0, MainSkin.Bitmap);
end;

Here the rectangle copyed from MainSkin is not Trasparent where the back
ground is black whay? (!). Have u a solution?
I must copy the rect from the MainSkin...like winamp system...

Thanks!
Have a good day!
Despx