alfresco-get-thumbnail-noderef.js

var node = search.findNode('<nodeRef>');
var assocname = '<thumbnailname>';
var nodeThumbnail = node.childAssocs['rn:rendition'].filter(function(assoc){
     return assoc.name == assocname;
})[0];

if(nodeThumbnail){
   logger.log(nodeThumbnail.nodeRef);
}