Not to offend. But, it's one of the unique qualities about the hub. I'm sorry. I've turned down others too. I would like to keep that one uniqueness, because even those who don't use them as profile pictures, they use other things, like you and I.
This gives my viewer something special that they don't get. If those who wear their pictures, which sometimes I do, want to change their pictures, it won't change my hub.
Their profiles, which are HubPages controlled, changes everything attached to them, whenever you change your profile picture. My hub, won't change.