Progress Bar/Loading Image on Visualforce Page by ActionStatus Tag in Salesforce

Some time we need to show some loading image or progress bar while we are doing some progress or calculation on page. In that case we can use <apex:actionStatus> tag to show loading image

<apex:page >

<apex:form >

    <apex:pageBlock id="PB1">

        <apex:pageBlockSection >

            <apex:commandButton status="waitStatus" value="Load Image" reRender="PB1"/>

        </apex:pageBlockSection>

    </apex:pageBlock>

<apex:actionStatus id="waitStatus" style="align:center;">      

            <apex:facet name="start">

                 <apex:image value="/img/loading.gif" title="Processing..."/>

             </apex:facet>

            <apex:facet name="stop"></apex:facet>

</apex:actionStatus>

</apex:form>

</apex:page>