Click on user CP on the left of the navigation bar
Click on Edit avatar
You now have 2 options, you can upload a picture from an existing site (i.e. yahoo images) or from your own computer.
If you choose option 1, from an existing site, copy and paste the URL into the space provided.
For option 2 to upload from your computer click on browse, and it will open your files on your desktop, choose your picture file (or wherever you have your pictures stored), and click on the one you want to use.
Note: it must be sized 100x100 pixels or 19.5 kb(so you may need to resize it first).
*Note: to resize your images to an "avatar" size, a photo hosting site (i.e.; photobucket) is helpful. You just create an account, load your picture, store it and then edit it(It really is as simple as follow the prompts). One of the size options in avatar, have it automatically resized and then save it back to your file and use that image, or use the URL for a direct link.