You can't do this from the iPad Forum app - you need to do this from the website and from a PC too (i.e. not from the iPad!).
When you're logged on from a PC there's a 'button' towards the top of the screen labelled 'User CP' (User Control Panel). If you click that a panel will open up down the left hand side of the screen that enables you to do all sorts of stuff - including changing that picture (avatar). There are contraints on the size of the picture, so you may need to reformat any picture that you have to begin with.
Tim