BuddyPress.org

At the moment, you really don’t. The blogs avatars are randomly created by gravatar.com. Blog avatars are coming in a future version of BP.

Because blogs can have multiple authors, and the first author isn’t necessarily the “main” one, there’s no mechanism in place to do that quite yet. It would be possible to fudge it as a custom function, but it won’t be that way in the core.