I coded a mod to override the default forum settings. I'll look into it tonight to make it show the photo if it exists, then the facebook photo, then twitter, then if none of them exist, show the default silhouette.
There. Done.
Ok, so this is how the avatars work now (because of my mod)
1. If you've uploaded a "photo" to your profile, this will show.
2. If there's no photo uploaded manually, but you've linked your Facebook account, your facebook photo will be used.
3. If you haven't done 1 or 2, but you've linked your Twitter account, your twitter photo will be used.
4. If you haven't done any of the above, your photo is the basic silhouette.