Ajax modelpopupextender

I am trying to use a modelpopup extender(from ajaxtoolkit) to display a popup in my aspx page. The modelpopup extender has been used in side a datalist.

<form id="form1" runat="server">
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
    <asp:DataList ID="DataList1" runat="server"
    RepeatColumns="4" RepeatDirection="Horizontal" 
    <ItemStyle BackColor="#EFF3FB" />      
  <div >
    <div >
   <asp:LinkButton ID="lnk_Caption" runat="server" CommandArgument = '<%# Eval("pictureID") %>'  CommandName = "Caption" > Caption 

<asp:ModalPopupExtender ID=“ModalPopupExtender1” runat=“server” TargetControlID=“lnk_Caption” PopupControlID=“DivCaption” OkControlID=“OkButton” >


<!–Pop up–>
<div runat=“server” id= “DivCaption” width=“150px” Height =“200px” >
<table border=“0” cellspacing=“0” cellpadding=“0” width =“150px” >
<td><textarea id=“Textarea1” name=“comment” runat=“server” class=“text_main_field” title=“E mail” onblur=“if(this.value==‘’) this.value=‘Caption here’;” onfocus=“if(this.value==‘Caption here’)this.value=‘’;” >Caption here</textarea></td>
<asp:Button ID=“OkButton” runat=“server” Text=“OK” onclick=“OkButton_Click” />
<asp:Button ID=“btnCancel” runat=“server” Text=“Cancel” onclick=“btnCancel_Click” />

I want to execute an update statement in OkButton_Click(). for that i need the pictureid. I am not able to get the picture id of the command argument given in the “lnk_Caption”. Pls help…