1. Click Settings at the top of any forum page.
  2. Edit Avatar (on the left)
  3. Check 'Use custom avatar' and then use Option 2 to upload the image from your computer.
  4. Click 'Save Changes' button.
If you want the avatar to be animated (assuming the file you have on your computer is animated) the maximum size of your custom image is 150 by 150 pixels or 68.4 KB (whichever is smaller).