First thing, you have to have picture no larger than 50X50 pixels.
Then click on the "User CP" button on the top right of the screen.
When the next screen comes up, click on the "User Options" link on the top near the middle of the screen.
At the bottom of that screen, click on the "Change Avatar" button.
On the bottom of the next page, there is a place to either enter a URL for an avatar somewhere on the web, or you can upload your own using the "Browse" button. Clicking that button with let ou choose an avatar that you have stored on our computer.
Then just click "Submit Modifications" and your done.
Any other questions, just give me a shout.