菜鸟做练习，做了一个最简单的类，怎么使这个类向上移动？？ (50分)

unit Unit1;

interface

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

type
TSprite = class
private
FBits, FInvBitmap: TBitmap;

public
constructor Create; virtual;
procedure LoadBits;
procedure Draw(Canvas: TCanvas); virtual;

end;

type
TForm1 = class(TForm)
procedure FormActivate(Sender: TObject);
private

{ Private declarations }
public
{ Public declarations }
end;

var
Form1: TForm1;
FTank: TSprite;
implementation

{$R *.DFM}

constructor TSprite.Create;
begin
inherited Create;
FBits := TBitmap.Create;
FInvBitmap := TBitmap.Create;
end;

procedure TSprite.LoadBits;
begin
FInvBitmap.LoadFromFile('a.bmp');

end;

procedure TSprite.Draw(Canvas: TCanvas);
begin
Canvas.Draw(1, 100, FInvBitmap);
end;

procedure TForm1.N2Click(Sender: TObject);
begin
FTank := TSprite.create;
ftank.LoadBits;
ftank.Draw(form1.Canvas);
end;

end.

点击菜单后窗体上出现一个图片，这时我想按向上的方向键可以使它向上移动，还要怎么写代码呢？

在onkeydown中擦除窗口重画