Good Question - was wanting to do the same thing. Would have been a bummer if all the contact photos were stuck on cartoon pics from a couple of weekends ago!
Could not find a sync option under accounts...
Tried going through the contact's facebook wall but that did not update it.
I think deleting and reestablishing the facebook account would do it but that seems extreme and likely to create multiple contracts that would need to be linked.
I suppose one could manually download the friend's new pic with the facebook app and link it to the contact.
Come to think of it, this would be better to do friend by friend...