Problem with Checkbox

unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, ExtCtrls;

type
TForm1 = class(TForm)
Button1: TButton;
Image1: TImage;
CheckBox1: TCheckBox;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
Form1: TForm1;
m,n:integer;

implementation

{$R *.dfm}
procedure lingkaran;
begin
form1.Image1.Canvas.Brush.Color:=clwhite;
form1.Image1.Canvas.Ellipse(m-77,n-77,m+77,n+77);
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
m:=75;
n:=57;
//buat kotak putih//
form1.Image1.Canvas.Brush.Color:=clwhite;
form1.Image1.Canvas.Rectangle(0,0,form1.Image1.Width,form1.Image1.Height);
//buat pohon//
//buat lingkaran//
if form1.CheckBox1.Checked=true then begin
lingkaran;
end;
form1.Image1.Canvas.Brush.Color:=clblue;
form1.Image1.Canvas.Ellipse(m-7,n-7,m+7,n+7);
end;


end.