Loading effect using JavaScript

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Javascript

I am trying to create loading effect when user submits the form. While my page is processing it should show loading image and when my page stops processing it should hide the loading image. How to acheive this on button click.

<script type="text/javascript" language="javascript">
function svalidate() {
       var category = document.getElementById('<%=dd_category.ClientID %>').value;
            if (category == "Select Category") {
                alert("Please select Category");
                return false;
            }
        }
function loading() {
            document.getElementById("load").style.display = 'none';
        }        
</script>

<div id="load">   
        <img style="margin-top:200px;" src="Images/loading.gif" alt="loading" />
        <br /><br />
        <h1 style="color:#8B7637;">Loading Please Wait..</h1>               
</div>

<asp:Button ID="but_go" runat="server" Text="Go" CssClass="gobuttonstyle" OnClientClick="return svalidate();" onclick="but_go_Click"/>