When you create an account and sign up on the forums from the website here, your given the opportunity to add an avatar during the sign up process. But if you login through Facebook your account could be created without an avatar.
Here's how you can add or change your avatar image.
First click on the "profile" tab near the top of the page.
That takes you to your profile page where you'll see an "edit" allowing you to edit your profile. Click the "edit" button on the right side of the page.
Once your on the edit page, you'll see tabs leading to different parts of your profile that can be edited. One of them is labeled "Avatar Image". Click the tab labeled "Avatar Image".
From there your given the options to change your avatar image or add an avatar image.