In iOS 6.0 Settings, you can "Update All Contacts" with Facebook and Twitter. According to the explanation in the phone this will update photos and user names for all matching contacts. In addition, you can allow Facebook to use your contacts.
I have a lot of contacts and many of them have already got pictures that I have chosen myself. The contacts (and pictures) are synced with my Google Apps account using Exchange.
If I do "Update All Contacts" with Facebook and/or Twittor or allow Facebook to use my contacts, will this overwrite existing pictures? (And thus change what is in my Google contacts?)
(For bonus points, please explain exactly what else will be changed.)