照教程做的图片浏览器，为什么浏览不了图片（菜鸟求助，就剩五分了，急）(5分)

因为毕业设计，需要用delphi制作多媒体课件，我是菜鸟．花了一个晚上时间照着教程做出了一个图片浏览器，但是不知道哪里出了问题，怎么也浏览不了图片．哪位前辈能告诉小弟其中的原因吗，改怎么修改代码．
下面是图片浏览器代码，编译能通过，但是功能运行不了：
unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, Grids, Outline, DirOutln, ExtCtrls, StdCtrls, FileCtrl, jpeg;

type
TForm1 = class(TForm)
DriveComboBox1: TDriveComboBox;
DirectoryListBox1: TDirectoryListBox;
FilterComboBox1: TFilterComboBox;
FileListBox1: TFileListBox;
Button1: TButton;
Button2: TButton;
Panel1: TPanel;
Image1: TImage;
procedure DriveComboBox1Change(Sender: TObject);
procedure DirectoryListBox1Change(Sender: TObject);
procedure FilterComboBox1Change(Sender: TObject);
procedure FormCreate(Sender: TObject);
procedure formdestroy(sender:tobject);
procedure FileListBox1Change(Sender: TObject);
procedure Button1Click(Sender: TObject);
procedure Button2Click(Sender: TObject);
procedure filelistbox1click(sender:tobject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
Form1: TForm1;
jpg:tjpegimage;
number:integer;

implementation

{$R *.dfm}

procedure displayimg;
var
ext:string[4];
begin
ext:=ansiuppercase(extractfileext(form1.FileListBox1.Items[number]));
form1.Caption:=extractfilename(form1.FileListBox1.Items[number]);
if(ext='.bmp')or(ext='.ico')then
begin
form1.Image1.Picture.LoadFromFile(form1.FileListBox1.Items[number]);
end
else if(ext='.jpg')then
begin
jpg.loadfromfile(form1.FileListBox1.Items[number]);
form1.Image1.Picture.Graphic:=jpg;
end;
form1.Button1.enabled:=true;
form1.Button2.enabled:=true;
if number=0 then
form1.button1.Enabled:=false;
if number=form1.filelistbox1.Items.count-1 then
form1.Button2.Enabled:=false;
form1.filelistbox1.ItemIndex:=number;
end;
procedure TForm1.DriveComboBox1Change(Sender: TObject);
begin
number:=0;
directorylistbox1.Drive:=drivecombobox1.drive;
filelistbox1.drive:=drivecombobox1.Drive;
filelistbox1.Directory:=directorylistbox1.Directory;
end;

procedure TForm1.DirectoryListBox1Change(Sender: TObject);
begin
number:=0;
filelistbox1.directory :=directorylistbox1.Directory;
end;

procedure TForm1.FilterComboBox1Change(Sender: TObject);
begin
number:=0;
filelistbox1.Mask:=filtercombobox1.Mask;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
filelistbox1.Mask:=filtercombobox1.Mask;
jpg:=tjpegimage.Create;
filelistbox1change(sender);
end;
procedure tform1.formdestroy(sender:tobject);
begin
jpg.free;
end;

procedure TForm1.FileListBox1Change(Sender: TObject);
begin
if filelistbox1.Items.Count>0 then
begin
displayimg;
end
else
begin
form1.button1.Enabled:=false;
form1.Button2.Enabled:=false;
end;
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
dec(number);
displayimg;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
inc(number);
displayimg;
end;
procedure tform1.filelistbox1click(sender:tobject);
begin
number:=filelistbox1.itemindex;
displayimg;
end;

end.