MS Paint like program

with Image1.Picture.Bitmap.Canvas do begin
  // Prepare Brush and Pen
  Brush.Style:=bsSolid;
  Pen.Color:=clBlack;
  Pen.Size:=1;
  // Make Background white (fill with a white solid brush)
  Brush.Color:=clWhite;
  FillRect(Rect(0,0,Image1.Width,Image1.Height);
  // draw a 2px thick red line
  Pen.Color:=clRed;
  Pen.Size:=2;
  MoveTo(0,0);
  LineTo(Image1.Width, Image1.Height);
  // draw a 2px blue rectangle filled with green
  Pen.Color:=clBlue;
  Brush.Color:=clGreen;
  Rectangle(20,20,50, 50);
end;

procedure TfrmUltraViewer.imgMainMouseDown(Sender: TObject;
  Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
  try
  {Prepare Brush and Pen}
  imgMain.Picture.Bitmap.Canvas.Brush.Style:= bsSolid;
  imgMain.Picture.Bitmap.Canvas.Pen.Color:= clBlack;
  {Begin Drawing}
  imgMain.Picture.Bitmap.Canvas.Pixels[mouse.cursorpos.X,mouse.cursorpos.Y]:= imgMain.Picture.Bitmap.Canvas.Pen.Color;
  finally
    imgMain.Free;
  end;
end;