This website automatically uses Gravatars, so all you need to do is create one for yourself and it will show up by your username. (Be sure to create your Gravatar with the email address you used to register.)
A Gravatar is a Globally Recognized Avatar. An “avatar” is an image associated with your email address that represents you online. It is a tiny picture (of your choice) that appears next to your name when you interact with any website that uses Gravatars. (My picture is a Gravatar.)
The Gravatar service is free. Here is a link where you can
Create a Gravatar
Important: Gravatar is now owned by WordPress. This means that you will be asked to sign up for a WordPress account as one of the steps for getting a Gravatar. (You will probably never use your WordPress account for anything else, but rest assured they will not bother you with emails or ads or anything else.) So, just do the simple sign up steps as requested (after you click the link above) and you will be able to get a Gravatar.