Just checked a few mod pages to see and thought it was a 'Curse Premium' feature now to have 'custom avatars'.
Or so I think, and then I check a few other sources and seems you have to tweak settings through Twitch?
I'm going to look into this to find out since I'm not sure.
Hope this doesn't make it's way to the MCForums.
I thought it might have been a universial thing on inappropriate/gif/licensed avatars but it doesn't seem that way.
In short though: No clue what so ever.