jqurey 选择器 取NULL值问题。

代码有点多，请大侠们耐心帮看下。 <script src="../jquery-ui-1.7.1/js/jquery-1.3.2.min.js" type="text/javascript"> </script>    <asp:DataList ID="dlCerificate" runat="server" DataSourceID="odsCerificate" RepeatColumns="4"
        RepeatDirection="Horizontal" Width="99%">
        <ItemStyle HorizontalAlign="Center" />
        <ItemTemplate>
            <img id="img" src='<%# Eval("cminpic","../Admin/Cerificate/images/{0}") %>' onclick="disPic('<%# Eval("cmaxpic","../Admin/Cerificate/images/{0}") %>'),test()"
                alt="点击查看大图" /><br />
            <label>
                <%# Eval("cname")%></label>
        </ItemTemplate>
    </asp:DataList>
    <asp:ObjectDataSource ID="odsCerificate" runat="server" SelectMethod="GetCerificateByTypeId"
        TypeName="BLL.CerificateManage">
        <SelectParameters>
            <asp:QueryStringParameter Name="tid" QueryStringField="typeid" Type="Int32" />
        </SelectParameters>
    </asp:ObjectDataSource>
    <div id="block" style="position: absolute; z-index: 2; display: none">
        <img id="picture" alt="" />
    </div>    <script type="text/javascript">
    // alert($("#img"));
   // alert($("#block"));
/*$("#img").click(function(){
  $("#block").animate({ 
    width: "90%",
    height: "100%", 
    fontSize: "10em", 
    borderWidth: 10
  }, 1000 );
});*/
 function test()
    {
    alert($("#block"));
      $("#block").slideToggle("slow");
       }
    function disPic(path)
    {
    alert(path);
    var divobj=document.getElementById("block");//获得div对象
    alert(divobj);
    var picobj=document.getElementById("picture");//获得img对象
    picobj.src=path;
    alert(picobj.src);
    divobj.style.top=event.clientY+"px";
    divobj.style.left=event.clientX+"px";
    divobj.style.display="block";
    }
   
    </script>
 alert($("#block"));
      $("#block").slideToggle("slow");
为什么会是NULL，我在另一个测试页面没问题，高手帮我看下哪出错了