Log in to the site, click 'My Account', then scroll down and under the heading 'My Preferences' click the link 'Click here to edit preferences..'
Where it says 'Avatar:' you'll see a 'Browse' button to the right. Click it to locate the picture/avatar from your computer that you wish to use.
Once you've selected the image you want to use, click the 'Save Changes' button and you're done.
You may have to click your browser 'Refresh' button once or twice to see your new avatar.
Let me know if you have any problems with it.
Enjoy!