Q:

facet image position not setting screen center its showing on top corner

<apex:pageBlock > 
   <apex:actionStatus id="actStatus" style="align:center;position:absolute;z-index:99999;"  >
     <apex:facet name="start">
        <img src="/img/loading.gif" />                      
    </apex:facet>
    </apex:actionStatus>

     <apex:outputText >Name</apex:outputText> 
     <apex:inputText value="{!name}"/>
     <apex:outputText >Gender</apex:outputText>           
     <apex:inputText value="{!gender}"/>   
     <apex:outputText >Email</apex:outputText> 
     <apex:inputText value="{!email}"/>
     <apex:commandButton value="Search" action="{!searching}" reRender="searchref" status="actStatus"/>
 </apex:pageBlock>

A:

You can create a div element with the background image in the center. Use this div inside the <apex:facet name="start"> to show the image in center. 
<apex:page controller="MyController">
    <style>
        #loadingDiv{
            position:fixed;
            top:0px;
            left:0px;
            width:100%;
            height:100%;
            background: url('https://upload.wikimedia.org/wikipedia/commons/d/de/Ajax-loader.gif') 50% 50% no-repeat #666; /* You can adjust the color */
            z-index:99999;
            opacity: 0.4; /* You can adjust the opacity */
            filter: alpha(opacity=40); /* For IE8 and earlier */
        }
    </style>
    <apex:form id="form">
        <apex:commandButton action="{!fetchDetails}" value="Fetch Details" rerender="form" status="status" />
        <apex:actionStatus id="status">
            <apex:facet name="start">
                <div id="loadingDiv"/>
            </apex:facet>
        </apex:actionStatus>
    </apex:form>
</apex:page>

