hello,
i am writing below code.

<apex:column headerValue="Product Picture">
         <apex:variable var="imageVar" value="{!abc.xyz.item__r.Url_Name__c}.jpg"/>
                 <apex:image url="{!URLFOR($Resource.KLM, imageVar)}" width="80" height="80"/>
   </apex:column>

here i am displaying image coming from static resource. But if image is not present in static resource then i want to display default image.. How to do it? i am newbie in SF.