That topic has already been covered, but I'll clear it up for you since I'm here.
It can be done in your Forum Profile settings, but right now, you can't, due to the fact that they've stopped people from being able to do that.
The site went down a little while back, due to an alleged attack, and ever since then, certain things have been restricted from being done. So right now, anyone without a picture, like you and myself, are not able to change it, until the owner comes back and fixes it, and to be honest, no-one knows when that is.
Hope this helped!