Add following code in master page immediately after opening form tag.
<asp:updateprogress runat="server" id="upMainProgress" xmlns:asp="#unknown">
<progresstemplate>
<div style="margin: 0 auto; width: 100%;">
<div id="load">
</div>
<div id="visibleDivProgress">
<img src="images/loading.gif" alt="Please Wait..." />
</div>
</div>
</progresstemplate>
</asp:updateprogress>