Do you mean the avatar (pic on left under your name and details) or in the signature (text at the bottom of your posts)?
If you go into your Profile and click on 'Forum Profile Information' you can upload an avatar straight from your PC. If it's a picture in your signature that you want then you use the same code stated above for posting pictures in threads using the 'img' tags.
For instance, my little HST at the bottom of my posts appears due to the following:-
Hope that helps