default value imagefield -> get from field collection item
I would look in to something like 

hook_node_presave

As a rough example I would have it work like this

function MY_MODULE_node_presave($node) {
     if(!isset($node-&gt;field_thumbnail)) {
          // load field collection and save that image to this image field
     }
}


I haven't tested this code but i think this would be the direction you would want to go in.