怎样动态切换SpeenButton的图标？

//pas
unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  ExtCtrls, ImgList, Buttons;type
  TForm1 = class(TForm)
    SpeedButton1: TSpeedButton;
    ImageList1: TImageList;
    Timer1: TTimer;
    procedure Timer1Timer(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
    FImageIndex: Integer;
  public
    { Public declarations }
  end;var
  Form1: TForm1;implementation{$R *.DFM}procedure TForm1.Timer1Timer(Sender: TObject);
begin
  SpeedButton1.Glyph := nil;
  ImageList1.GetBitmap(FImageIndex, SpeedButton1.Glyph);
  if FImageIndex < ImageList1.Count then
    Inc(FImageIndex)
  else FImageIndex := 0;
end;procedure TForm1.FormCreate(Sender: TObject);
begin
  FImageIndex := 0;
end;end.//dfm
object Form1: TForm1
  Left = 192
  Top = 107
  Width = 544
  Height = 375
  Caption = 'Form1'
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'MS Sans Serif'
  Font.Style = []
  OldCreateOrder = False
  OnCreate = FormCreate
  PixelsPerInch = 96
  TextHeight = 13
  object SpeedButton1: TSpeedButton
    Left = 128
    Top = 72
    Width = 23
    Height = 22
  end
  object ImageList1: TImageList
    Left = 240
    Top = 56
    Bitmap = {
      494C01010A000E00040010001000FFFFFFFFFF00FFFFFFFFFFFFFFFF424D3600
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      000000BA0000005D0000002E800000175000000BA8000005C800000284000001
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      00000000000000000000000000000000000000000000FFFFFFFFFFFFFFFFFFFF
      FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF0000000000000000000000000000
      0000000000000000000000000000000000000000000000000000000000000000
      0000000040002000080001012000674E0001000000007E200000002000000000
      000000000000000000000000000000000000FFFFFF0000000000000000000000
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  end
  object Timer1: TTimer
    OnTimer = Timer1Timer
    Left = 168
    Top = 56
  end
end