[Asinkrit]

Вот такой метод решит твою проблему.

Код:

procedure TForm1.FormPaint(Sender: TObject);
begin
  with Image1.Canvas do
    begin
    Pen.Style:=psSolid;
    Pen.Color:=clBlack;
    MoveTo(0,0);
    LineTo(Width,Height);
    end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  DoubleBuffered:=true;
end;

procedure TForm1.FormResize(Sender: TObject);
begin
  Image1.Picture.Bitmap.Height:=Height;
  Image1.Picture.Bitmap.Width:=Width;
end;