Hi Kitson,
If you want to change your avatar, log in to the character you want to be, equip anything you want to show up, and then type /char . This will update your avatar - and next time you use the log in form on the forum, it'll download the updated version.
I know, it's a little awkward so far - but I'll add an actual control (such as a button you can click) that will be easier to use - until then, just follow the steps above to update it. ^^
Toby