Hello and welcome to the Sponge community!
It is indeed possible to change your avatar. We are in the proces of switching over to a SSO-service for all our sites, which can be found here. Log in with the same username and password as on the forums, then you should be able to set your avatar, or even use Gravatar if you so desire. However, I believe the service has had a few small problems with syncing avatars, so it might take a while before the change is reflected here on the forums. It will come in time, though!