Click
User CP, then on the left there should be a link stating:
Edit Avatar. Then once you are there either click on the standard forum avatars or click on the 'Use Custom Avatar' button, then 'Browse' and then choose which picture to upload(while making sure that the maximum size of your custom image is 100 by 100 pixels or 97.7 KB [whichever is smaller]), then press '
Save changes' at the bottom of the screen. Then you are done.