<script>
        // To show / hide waiting-flower icons
        function setVisibility(id, isVisible){
            document.getElementById(id).style.visibility= isVisible ? 'visible' :  'hidden';
            
        }
        
        function show(id){setVisibility(id,true);}
        function hide(id){setVisibility(id,false);}
      
</script>
 
<apex:actionstatus id="pleasewait" onstart="show('img_status_product_select');" onstop="hide('img_status_product_select');">        
<img height="20px" id="img_status_product_select" src="%7B%21$Resource.AjaxAnimation%7D" tyle="visibility:hidden" width="20px">
 
</apex:actionstatus>

http://force.siddheshkabe.co.in/2009/10/displaying-please-wait-ajax-animation.html

Credits to Siddhesh 'Ravan' Kabe

Displaying please wait ajax animation on Visualforce page

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