You can use code as below when
using Image as Loading
<asp:UpdateProgress id="updateProgress" runat="server">
<ProgressTemplate>
<div style="position: fixed; text-align: center; height: 100%; width: 100%; top: 0; right: 0; left: 0; z-index: 9999999; background-color: #000000; opacity: 0.7;">
<asp:Image ID="imgUpdateProgress" runat="server" ImageUrl="~/images/ajax-loader.gif" AlternateText="Loading ..." ToolTip="Loading ..." style="padding: 10px;position:fixed;top:45%;left:50%;" />
</div>
</ProgressTemplate>
</asp:UpdateProgress>
using Text as Loading
<asp:UpdateProgress id="updateProgress" runat="server">
<ProgressTemplate>
<div style="position: fixed; text-align: center; height: 100%; width: 100%; top: 0; right: 0; left: 0; z-index: 9999999; background-color: #000000; opacity: 0.7;">
<span style="border-width: 0px; position: fixed; padding: 50px; background-color: #FFFFFF; font-size: 36px; left: 40%; top: 40%;">Loading ...</span>
</div>
</ProgressTemplate>
</asp:UpdateProgress>