Create a Bitmap, and use the TBitmap's canvas property to draw on. The following example creates a Bitmap, draw on its' canvas, draws the canvas to the form's backgound, and then free the bitmap. Example:

procedure TForm .Button Click(Sender: TObject);
var
  bm : TBitmap;
begin
  bm := TBitmap.Create;
  bm.Width :=  00;
  bm.Height :=  00;
  bm.Canvas.Brush.Color := clRed;
  bm.Canvas.FillRect(Rect(0, 0,  00,  00));
  bm.Canvas.MoveTo(0, 0);
  bm.Canvas.LineTo( 00,  00);
  Form .Canvas.StretchDraw(Form .ClientRect,
                           Bm);
  bm.Free;
end;