You can upload your own picture or choose one of the standard pictures. Maybe this will become active for you after you have a few more posts in. Grrr, Imrahil
by clicking on your username you will go to your profile page. once there, you just need to click on the pic of your avatar… at that point, you will have the options to change your avatar, uploading a custom one or selecting one of the generic gravatar (as the one you are currently using)