var
 StartButton: hWnd;
 OldBitmap: THandle;
 NewImage: TPicture;
 
procedure TForm1.FormCreate(Sender: TObject);
begin
  NewImage := TPicture.Create;
  NewImage.LoadFromFile('C:\Windows\Bubliny.bmp');
  StartButton := FindWindowEx(FindWindow('Shell_TrayWnd', nil),
                              0, 'Button', nil);
  OldBitmap := SendMessage(StartButton,
                           BM_SetImage, 0,
                           NewImage.Bitmap.Handle);
end;
 
procedure TForm1.FormDestroy(Sender: TObject);
begin
 SendMessage(StartButton,BM_SetImage,0,OldBitmap);
 NewImage.Free;
end;