Gravatar is an avatar-hosting service, that's now part of Wordpress. (So you need to sign up for a Wordpress account). Once you've signed up, you can upload your chosen avatar to the Gravatar/Wordpress website. When AB's server displays pages here, it contacts Gravatar/Wordpress to see if there's an avatar registered to the same email address as a user posting on AB has. If so, it then calls up that avatar and displays it next to the user's screen name.
So it's vital that the email address you use to register with Gravatar/Wordpress is the same one that you used to register with this site. (It's the only way that your avatar can be linked to your AB account). If you're asked to give a 'rating' to your avatar on Gravatar/Wordpress, it's also essential to choose 'G'. (The ratings system is based upon US film classifications, with 'G', for 'general', being the equivalent of a British 'U' film classification).
See here:
https://www.theanswerbank.co.uk/AB-Editors-Blog/article/what-is-an-avatar/