...
private
  OutCanvas: TCanvas;
  OldWinProc, NewWinProc: Pointer;

procedure NewWinProcedure(var Msg: TMessage);
...

procedure TMainForm.FormCreate(Sender: TObject);
begin
  NewWinProc := MakeObjectInstance(NewWinProcedure);
  OldWinProc := Pointer(SetWindowLong(ClientHandle,
    gwl_WndProc, Cardinal(NewWinProc)));
  OutCanvas := TCanvas.Create;
end;

procedure TMainForm.NewWinProcedure(var Msg: TMessage);
var
  BmpWidth, BmpHeight: Integer;
  I, J: Integer;
begin
  // default processing first
  Msg.Result := CallWindowProc(OldWinProc,
    ClientHandle, Msg.Msg, Msg.wParam, Msg.lParam);

  // handle background repaint
  if Msg.Msg = wm_EraseBkgnd then
  begin
    BmpWidth := MainForm.Image1.Width;
    BmpHeight := MainForm.Image1.Height;
    if (BmpWidth <> 0) and (BmpHeight <> 0) then
    begin
      OutCanvas.Handle := Msg.wParam;
      for I := 0 to MainForm.ClientWidth div BmpWidth do
        for J := 0 to MainForm.ClientHeight div BmpHeight do
          OutCanvas.Draw(I * BmpWidth, J * BmpHeight,
            MainForm.Image1.Picture.Graphic);
    end;
  end;
end;

procedure TMainForm.FormDestroy(Sender: TObject);
begin
  OutCanvas.Free;
end;