The easiest way to post a new one from your User Control Panel (UserCP) where you can find many account management features. Here is a link direct link to the avatar pag:
If you don't have a copy of your old avatar and need for us to pull it from the old software, we can do this manually. Please open a help desk ticket by using the Contact Us for at the bottom of the page and we will get you hooked up.
Here is a link to the Contact Us page for quick access: