try this:
.aspx file:
<cc1:modalpopupextender id="mp1" runat="server" popupcontrolid="Panel1" targetcontrolid="btnShow" xmlns:cc1="#unknown">
CancelControlID="btnClose" BackgroundCssClass="modalBackground">
</cc1:modalpopupextender>
<asp:panel id="Panel1" runat="server" cssclass="modalPopup" align="center" style="display:none" xmlns:asp="#unknown">
This is an ASP.Net AJAX ModalPopupExtender Example<br />
<asp:image id="img1" runat="server">
ImageUrl='~/Image/abc.jpg'></asp:image>
<asp:button id="btnClose" runat="server" text="Close" />
</asp:panel>
.aspx.cs file: (Button Click Event)
void Btn_Click(Object sender, EventArgs e)
{
mp1.show();
}