Hi Fred,
When you make a post (assuming you use the reply button) or when posting a new topic - there is a box which says 'Message Icon'. The usual icon is 'General' but if you click the arrow you can choose the icon that best suits your topic.
I have a small dog at the moment who really does not like cats (pretty much the usual with small dogs) so I have no cat at this time but I still have a soft spot for cats, thus the avatar.
Tweakie.