Feature is disabled however if you contacted an admin. Maybe they will change it for you but I have asked before in past to have it changed to something more updated and different but did not change for me. Maybe will come to allowing it who knows no doubt will stay disabled. Solution to this would be to update smf forum version and sort a lot of problems that it has on the first ever release of it.
As far as know it's hard for user who already have an avatar to have it changed.
It's impossible for those of us who don't have any.
We need to wait until the new forum is released.