Setting contact picture from linked contact I use Local Sync as my main address book. Several of my contacts are linked to different address books (e.g. LinkedIn, Skype, Whatsapp, Viber).

In my case, I found a contact having a very beautiful photo in Viber but I chose to set an old ugly picture in my address book

The question is: how do I tell Android to select the contact picture to display for a given contact not from Gallery/Camera apps but from that same contact's linked Viber account?

The question can be generalized and rephrased to cover not only Viber: 

how do I tell Android to select the contact picture to display for a given contact not from Gallery/Camera apps but from one of that same contact's linked accounts?
I just found the answer, which is a little tricky.
In general
When you need to set a default picture t oa contact who is linked to others, simply press MENU and go to Defaults menu. There you have the Caller ID setting in which you can choose the picture being displayed on call.
In the Viber case
That is not possible, since Viber doesn't actually create linked accounts, unlike Whatsapp. If the application that allows people to set a photo to themselves and manages contacts doesn't implement writing to address book that won't be possible