In addition to standard avatars you select form the gallery, MFO has special "stats" avatars for Aoe3 (left), and Xbox Live (right).
To enable Xbox Live avatar, go the usual Home/Your Profile/Show Gallery way, select Private category and click Go. If you have Xbox Live account and your MFO name is the same as your gamertag, you'll see image with your Live stats. Select it, press select avatar, and then press submit on the profile page.
If your gamertag is different from your MFO name, you need first add your game tag to your MFO settings line. (The one above big signature box.) Type XboxLiveGameTag=yourgamertag there, separate it with space if something is there already. Submit changes in your profile. Go to Your Profile again and now you will see your avatar in the private category.
Same deal with AoE3 stats, but the syntax is AoE3Name=yourESO2nickname.
F-Zero avatar pictures your favourite ship. Adding that is a two step process. First you add F0FavouriteShip=YourFavouriteShip to MFO settings line and submit, then go to Your Profile again and select the avatar.