The same avatar I've used and never changed in every SMBX since I've been on the community, a stuffed dog keychain from my childhood. Brings me back some memories as it got lost last year.
My avatar is Vellisia, a dragon character from the first book I wrote and published, Ssalia and the Dragons of Avienot. I drew this particular picture on Drawception over a year ago to use it there but I've also been using it elsewhere since. I could put a bigger version but I couldn't be bothered to manually resize it so I just used the shrunken image from Drawception. Velli has been representing me pretty much everywhere in the past years, though she wasn't created for that purpose.
My avatar is still Vitellary, professor of the D.S.S. Souleye, in the room that he typically stands in. I don't think I'm going to ever remember to change it. Unless I look at this post again.