Try using this :
<apex:repeat value="{!Case.attachments}" var="att">
<apex:panelgrid columns="1">
<h1>Image<h1>Image</h1>
<apex:image url="{!URLFOR($Action.Attachment.Download, att.id)}"/>
</apex:panelgrid>
</apex:repeat>
Let me know if it works for you. Thanks