I've added Facebook to my accounts, and now Stack Overflow is displaying the photo from my Facebook profile instead of Gravatar.
Can I still have an option to use Gravatar?
If you log in to facebook.stackoverflow.com using Facebook, we'll pull down your Facebook profile picture auto-magically. Once connected, changes to your Facebook profile picture will be reflected on Stack Overflow
As your facebook.SO account and regular SO account are the same thing, changing the profile picture on either place reflects on both sites.
You can toggle back to Gravatar (and then back to Facebook, if you so desire), using the link underneath your avatar when editing your profile. When you turn the Facebook image off, it stays off. The trigger is the first login, basically.
Switching back to Gravatar was going to be possible at launch, but then there's a minor earthquake on the east coast; held up that feature*.
Anyway, we've pushed it now.
*Technically, it kept me from
hg push-ing it in time since I had to switch locales and machines.