Problem in showing a url(photo) from PostBackUrl property of asp:ImageButton

Hi,

I have a row in my asp:DetailsView  to show thumb image of some product and want to show the full size photo when the thumbImage is clicked.
I write the following code

          <asp:TemplateField HeaderText="Image">
                 <ItemTemplate>
                           <asp:ImageButton ID="ImageButton1"  runat="server" ImageUrl='<%# Eval("PictureURL", @"ProductImages\thumb_{0}") %>'
                             PostBackUrl='<% ...