What you're actually looking for is your "avatar".
Click on "User PC" > "Settings & Options" > "Edit Avatar". In the box "Custom Avatar", click on [Choose File] under Option 2, and you then have to locate your original picture on your computer's window (You'll know what I'm talking about once you see it).
Once you highlighted your picture's name, click on [Choose] and [Save Changes] and you should be set.
Hope it helps!