[CoolNard]

Well, just in case you return, the option for customizing your avatar is right below all those default avatars - with the prior sub-heading as "Custom Avatar" under the default avatar sub-heading. Oh, silly me, the main heading is "Edit Avatar", which you can easily gain access to by going to your "User CP".

After you've done that, enable your "Use Custom Avatar" activation by clicking on the white circular trigger on the left of it. Furtherdown, you'll see two options with their respective instructions briefly provided.

However, if all don't work, either your computer is faulty (it hates you! Just kidding, lol) so you're gonna have to fix it up, or consult a mod due to a probable error on the site itself.