You must be logged in to customize your user icon. Click on "My account" at the top right of the page. Then click on the "Edit Account" link. This will take you to the Account Settings page.
Scroll down to "Picture" and click the browse button to select the desired icon from your computer. The avatar should be at least 250 pixels wide by 250 pixels high. The file should be no larger than 500 kilobytes.
After you have selected your file, scroll to the bottom of the page and click Submit to upload the file and update your settings.