use lik this ,
this one have the "x" symbol for closing the tab
<asp:Button runat="server" ID="btnShow" Style="display: none" />
<ajax:ModalPopupExtender ID="ModalPopupExtender2" runat="server" TargetControlID="btnShow"
PopupControlID="panelID" BackgroundCssClass="modalBackground" OkControlID="imgID">
</ajax:ModalPopupExtender>
<asp:Panel runat="server" ID="panelID" CssClass="modalPopup">
<div class="inner" id="div1">
<ol>
<li style="font-weight: bold; color: White; background-color: #5D7B9D;">
<img id="imgID" src="../../images/cal_close.gif" alt="" align="right" />Edit_Comments
</li>
</ol>