Can I place the authors avatar (not current_user) in header?

I found a blog post here at wpmudev that explained how to place an avatar in your theme:

    global $current_user;
        echo get_avatar( $current_user->ID, 128 );

I added it to the header.php and it works. However, what if you wanted to show the authors avatar (multisite) instead of the current_user's avatar so that the avatar would always display in the header and not only to the "current_user" as it does with the above code? The above code only displays to the current logged in users and not regular visitors viewing the site.

Is that possible?

I was looking for a theme that had that built in already to display the author avatar and figured I may be able to see how its done by examining the code but couldn't find one yet.