이미지가 정사각형이어야 합니다...
전 이쁜 김하늘을 넣어서 씀 ㅡ.ㅡ
pravite
     FClientInstance,
     FPrevClientProc: TFarProc;
    procedure ClientWndProc(var Message: TMessage);

procedure TF_Base_Main.FormCreate(Sender: TObject);
begin
   FClientInstance := MakeObjectInstance(ClientWndProc);
   FPrevClientProc := Pointer(GetWindowLong(ClientHandle, GWL_WNDPROC));
   SetWindowLong(ClientHandle, GWL_WNDPROC, LongInt(FClientInstance));
end;

procedure TF_Base_Main.ClientWndProc(var Message: TMessage);
var
   MyDC: hDC;
   Ro, Co: Word;
   Images : TImage;
begin
   Images := TImage.Create( Application );

   Images.Picture.Bitmap := Kimhanul.Picture.Bitmap;

   with Message do
      case Msg of
         WM_ERASEBKGND:
         begin
            MyDC := TWMEraseBkGnd(Message).DC;
            for Ro := 0 to ClientHeight div Images.Picture.Height do
               for Co := 0 to ClientWIDTH div Images.Picture.Width do
                  BitBlt(MyDC, Co * Images.Picture.Width, Ro * Images.Picture.Height,
                  Images.Picture.Width, Images.Picture.Height,
                  Images.Picture.Bitmap.Canvas.Handle, 0, 0, SRCCOPY);
            Result := 1;
         end;
      else
         Result := CallWindowProc(FPrevClientProc, ClientHandle, Msg, wParam, lParam);
      end;
end;

자료실에 MDI 폼에 그림넣는 콤포넌트가 있어요.
물론 bmp만 되는게 쬠 아쉽지만...
쉽게 사용하실 수 있으실 거예요.