It's in the user preferences. Get there by clicking the third button from the left in the navigation at the top. It's called an Avitar. You can choose one of the avitars provided by TT or you can provide your own using the guidlines set by TT.
In order to use an avatar you must have at least 10 quality posts. Spam messages in order to reach 10 shall be simply deleted. Don't fret though, there are thousands of threads to put some good posts into. 🙂