Will updating contacts with Facebook or Twitter overwrite pictures? In iOS 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.

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.)
Facebook contact sync will add photos to contacts that don't have any pictures assigned and leave those that have them already unchanged.

Sources: on Facebook contact sync and review of the feature on iLounge with the answer to the question.