"How can I show the thumbnail of the image?" or "How can I obtain xxxx rendition?", Is quite simple, but I believe that is not documented.
This is a sample for a thumbnail:
<af:image source="#{WCAppContext.applicationURL}/content/conn/UCM/uuid/dDocName%3a#{node.propertyMap['dDocName'].value.stringValue}?rendition=thumbnail"
id="i1"/>
Where "UCM" is the name of your content connection inside the WebCenter Portal application.
I hope this helps you on your templates development.