Are you referring to the avatar picture that would appear under your username/above your join date/post count on the left hand side of your posts?
If so, then all you need to do is simply click on User CP (found at the top of the page on the left hand side), then select Edit Avatar under Settings & Options in Your Control Panel (this is located in the left hand column on the page). You can then choose to either upload an image from your favourite photo sharing site or upload it directly from your computer. Be sure to click save and you're all set!
HTH and if you have any more questions, please feel free to drop me a note and I'll try to help further