procedure TForm1.PaintBox1Paint(Sender: TObject; Canvas: TCanvas);
...
var i:integer;
begin
Randomize;
Paintbox1.Canvas.Brush.Color:=clWhite;
Paintbox1.Canvas.Rectangle(0,500,500,0);
Paintbox1.Canvas.MoveTo(1,0);
Paintbox1.Canvas.LineTo(1,500);
Paintbox1.Canvas.Pixels[xx,yy]:=clBlack;
...
end;
end.