Loading GIF for the portal

I want to get the loading gif animation for my webportal. I have one problem here. From page1.html: I am using a select dropdown when gets selected, it redirects to another Page2.html but before that it goes to page2.xconf which runs a groovy script where the processing takes time. So in this scenario, I cannot use regular loading gif in the market, because my Page2.html will not be loaded until that groovy script finishes processing.

<select class="reportselect">   
    <option id="none" value="none"></option>
    <option id="categoryMix.rptdesign" value="$home$apphome/adminreports/showreport.html?reportfile=categoryMix.rptdesign">Catagory Mix</option>
    <option id="dealerMix.rptdesign" value="$home$apphome/adminreports/showreport.html?reportfile=dealerMix.rptdesign">Dealer Mix</option>
    <option id="distributorMix.rptdesign" value="$home$apphome/adminreports/showreport.html?reportfile=distributorMix.rptdesign">Distributor Mix</option>
    <option id="monthlyMix.rptdesign" value="$home$apphome/adminreports/showreport.html?reportfile=monthlyMix.rptdesign">Monthly Mix</option>
    <option id="orderMix.rptdesign" value="$home$apphome/adminreports/showreport.html?reportfile=orderMix.rptdesign">Order Mix</option>

    </select>


<script>        /* on Dropdown change, loading a the new report */
    $('.reportselect').change(function() {
    var url = $(this).val();
    if(url && url !="none")
        {
            var win = window.open(url,"_self","","");

        }

    }); 
</script>

How should I accomplish this task ?

Thanks