Setting contact picture from linked contact
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