On ABXD Mini, I have an email field, a public email checkbox, and a gravatar checkbox. It has no avatar linking or uploading, only gravatars. If you provide an email address, no matter the public/private switch, and enable gravatar, it'll try to use that email address. Hashed, of course, as Gravatar wants. If your email isn't public, your profile page won't show it.

So "[email protected]" → "d0bd59922f5096b2c559f2b3a5d6ded9" →

If I wanted to have both gravatars and moods, I'd probably take the current system (upload your primary ava and any moods) and add a "use gravatar" checkbox. If you pick a mood, it'll use the one you uploaded, if you pick "-Normal Avatar-" it'll use... either the uploaded pic, your gravatar, or none if you so happen to have no normal avatar set. That's what I'd do.

If gravatars are enabled && email is set, use gravatar src, else if picture is set, use picture src, else print nothing.

bam you're done. If it's overcomplication, it's not a whole lot.