I was in a similar situation. My Dad is linked via his google account to my phone, which displays his phone # and email address. He also has a Facebook account (lol dad?) but he doesn't have a picture associated with his Facebook profile. The strange thing was, when I had everything linked to him (Dad = google account + facebook), even if I changed the display picture for him, facebook would always override it with that blank silhouette of a person. Got really annoying. I'd rather see my Dad's picture than nothing. So I unlinked his facebook name, and then deleted the contact. Problem solved.
Possibly kinda the same thing going on with you?
Facebook seems to override whatever pictures you want anyway. If I choose a picture to associate to my girlfriend, but I have her associated with facebook, it'll eventually revert back to the facebook picture. Doh.