Заполнение изображением MDI-формы 2

...
private
{ Private declarations }

procedure WMIconEraseBkgnd(var Message: TWMIconEraseBkgnd);
  message WM_ICONERASEBKGND;
...

USES MdiWal1u;

procedure TForm2.WMIconEraseBkgnd(var Message: TWMIconEraseBkgnd);
begin
  TForm1(Application.Mainform).PaintUnderIcon(Self, Message.DC);
  Message.Result := 0;
end;

...
{ Private declarations }
bmW, bmH: Integer;
FClientInstance,
FPrevClientProc: TFarProc;

procedure ClientWndProc(var Message: TMessage);
public
    procedure PaintUnderIcon(F: TForm; D: hDC);
    ...
      procedure TForm1.PaintUnderIcon(F: TForm; D: hDC);
    var

      DestR, WndR: TRect;
      Ro, Co,
        xOfs, yOfs,
        xNum, yNum: Integer;
    begin

      {вычисляем необходимое число изображений для заливки D}
      GetClipBox(D, DestR);
      with DestR do
      begin
        xNum := Succ((Right - Left) div bmW);
        yNum := Succ((Bottom - Top) div bmW);
      end;
      {вычисление смещения изображения в D}
      GetWindowRect(F.Handle, WndR);
      with ScreenToClient(WndR.TopLeft) do
      begin
        xOfs := X mod bmW;
        yOfs := Y mod bmH;
      end;
      for Ro := 0 to xNum do
        for Co := 0 to yNum do
          BitBlt(D, Co * bmW - xOfs, Ro * bmH - Yofs, bmW, bmH,
            Image1.Picture.Bitmap.Canvas.Handle,
            0, 0, SRCCOPY);
    end;

    procedure TForm1.ClientWndProc(var Message: TMessage);
    var
      Ro, Co: Word;
    begin

      with Message do
        case Msg of
          WM_ERASEBKGND:
            begin
              for Ro := 0 to ClientHeight div bmH do
                for Co := 0 to ClientWIDTH div bmW do
                  BitBlt(TWMEraseBkGnd(Message).DC,
                    Co * bmW, Ro * bmH, bmW, bmH,
                    Image1.Picture.Bitmap.Canvas.Handle,
                    0, 0, SRCCOPY);
              Result := 1;
            end;
          WM_VSCROLL,
            WM_HSCROLL:
            begin
              Result := CallWindowProc(FPrevClientProc,
                ClientHandle, Msg, wParam, lParam);
              InvalidateRect(ClientHandle, nil, True);
            end;
        else
          Result := CallWindowProc(FPrevClientProc,
            ClientHandle, Msg, wParam, lParam);
        end;
    end;

    procedure TForm1.FormCreate(Sender: TObject);
    begin

      bmW := Image1.Picture.Width;
      bmH := Image1.Picture.Height;
      FClientInstance := MakeObjectInstance(ClientWndProc);
      FPrevClientProc := Pointer(
        GetWindowLong(ClientHandle, GWL_WNDPROC));
      SetWindowLong(ClientHandle, GWL_WNDPROC,
        LongInt(FClientInstance));
    end;