I selected mine when I was asked to create my profile etc. when you sign up.
I guess Avatar means picture. Or at least that's what it means here. It's easy to do. Just click on 'My Account' at the top of the screen, and when you get to where you can change your profile, scroll down to where it asks for your Chat Avatar. That's the one you see here on the messages. Find a pic in your computer and put it into the box. You can also pick a different picture that comes up if someone wants to look at your profile. Good luck, and have fun with it.
If you need to find a pic to use as an avatar do a search on google for avatars. I'm sure you'll find something.
Thank You!
