You can't change your avatar from the Forum app - to do that you need to go to ipadforums.net and then click on the 'User CP' (control panel) button. A column to the left of the page will open up and you can add an avatar from there - as well as modify any of your Home page settings.
Well, in a way. But you can always open a thread, tap on the bold title and iPF will open the same thread as a website, which then let's you edit any settings the iPF app may be missing.