Go into your profile and click on the tab marked "Avatar." You will need to use an image that is saved to your computer. To add this image, click on the "Browse" button and locate the image you want to use as your avatar. Then click on "Update" and "Save Changes." It will take a few minutes for your avatar to appear.
Be sure to click off "Yes" beside "Enable Avatar." If you don't do this, people won't be able to see your avatar.