type
  .... = class(TForm)
    ....
      procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    ....
    private
    FHBrush: HBRUSH;
    FCover: TBitmap;
    FNewClientInstance: TFarProc;
    FOldClientInstance: TFarProc;
    procedure NewClientWndProc(var Message: TMessage);
    ....
    protected
    ....
      procedure CreateWnd; override;
    ....
  end;

  .....

implementation

{$R myRes.res} //pесуpс с битмапом фона

procedure.FormCreate(...);
  var
  LogBrush: TLogbrush;
begin
  FCover := TBitmap.Create;
  FCover.LoadFromResourceName(hinstance, 'BMPCOVER');
  with LogBrush do
  begin
    lbStyle := BS_PATTERN;
    lbHatch := FCover.Handle;
  end;
  FHBrush := CreateBrushIndirect(Logbrush);
end;

  procedure.FormDestroy(...);
    begin
      DeleteObject(FHBrush);
      FCover.Free;
    end;

    procedure.CreateWnd;
    begin
      inherited CreateWnd;
      if (ClientHandle <> 0) then
      begin
        if NewStyleControls then
          SetWindowLong(ClientHandle, GWL_EXSTYLE, WS_EX_CLIENTEDGE or
            GetWindowLong(ClientHandle, GWL_EXSTYLE));

        FNewClientInstance := MakeObjectInstance(NewClientWndProc);
        FOldClientInstance := pointer(GetWindowLong(ClientHandle, GWL_WNDPROC));
        SetWindowLong(ClientHandle, GWL_WNDPROC, longint(FNewClientInstance));
      end;
    end;

    procedure.NewClientWndProc(var Message: TMessage);

      procedure Default;
      begin
        with Message do
          Result := CallWindowProc(FOldClientInstance, ClientHandle, Msg,
            wParam,
            lParam);
      end;

    begin
      with Message do
      begin
        case Msg of
          WM_ERASEBKGND:
            begin
              FillRect(TWMEraseBkGnd(Message).DC, ClientRect, FHBrush);
              Result := 1;
            end;
        else
          Default;
        end;
      end;
    end;