1. upload your picture to the ifish server, or upload your picture to some location (like your web area provided by your service provider) that is reachable via the web.
Since space is money, it is better to use you own space if you can.
2. Press the "PROFILE" button
3. Click on the » View/Update Profile « link
4. Scroll down to the "Avatar Image" section, and
click on the "Select a different Avatar Image" link
5. In the text box on the bottom, type in the full address to the picture you uploaded.
Done.