想做一个从网站源代码提取图片地址，然后进行浏览的软件；可是不知怎么实现上一张、下一张的效果，具体代码如下：
unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, IdBaseComponent, IdComponent, IdTCPConnection,
  IdTCPClient, IdHTTP, ExtCtrls, GIFImage;

const
   num=35;

type
  TForm1 = class(TForm)
    IdHTTP1: TIdHTTP;
    Image1: TImage;
    procedure FormCreate(Sender: TObject);


  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  IMS:TMemoryStream;
  i,j:integer;
  k:integer=35;//一共提取了35张图片地址
  tmp,url:String;
  Gif: TGIFImage;
  function SplitPicUrl(LeftStr,RightStr,Data: string):TStringList;


implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
IdHTTP1:=TIdHTTP.Create(nil);
tmp:=IdHTTP1.Get('http://www.');
url:=SplitPicUrl('<option value="','" selected="selected">',tmp)[k];//读取第k+1个图片的地址并写入url中
IMS:=TMemoryStream.Create;
IdHTTP1.Get(url,IMS);
Image1.Picture.Assign(nil);
Gif:= TGIFImage.Create;
IMS.Position:=0;
Gif.LoadFromStream(IMS);
Image1.Picture.Assign(Gif);
IMS.Free;
Gif.Free;
IdHTTP1.Free;
end;

  function SplitPicUrl(LeftStr,RightStr,Data: string):TStringList;//将Data中LeftStr与RightStr之间的字符全部截取出来
      var
          i,j,k: integer;
          tmp: string;
      begin
           result := TStringList.Create;

           for k:=0 to 35 do//提取了35个图片地址
           begin
           i:=pos(LeftStr, Data);
           j:=pos(RightStr,Data);
           tmp:=copy(Data, i + Length(LeftStr), j - i- Length(LeftStr));
           result.Add(tmp);
           Delete(Data, 1, j - 1 + Length(RightStr));
           end;

      end;

end.

程序可以显示出网站最新的图片，但想添加两个按钮实现上一张、下一张，就捉急了！小白一枚，网上各种搜也找不到解决，只能寄希望于各位大神了！！拜谢！

自己顶一下，比如加了这一段代码，老是到IMS.Position:=0;就出错；百思不得其解！！

procedure TForm1.Button1Click(Sender: TObject);
begin
k:=k-1;
url:=urllist[k];
IdHTTP1.Get(url,IMS);
IMS.Position:=0;
Gif.LoadFromStream(IMS);
Image1.Picture.Assign(Gif);
IMS.Free;
Gif.Free;
IdHTTP1.Free;
end;

确定下IMS对象有值？？