To change your avatar head to the account settings page. Once this page loads you'll see an option to update your avatar:

When you click the 'Update Avatar' button you'll have a box pop up:

You now have the choice of using one of the cool avatars we've uploaded that represent the current best players on each team's roster, as well as some 'legend' avatars and team logos. You can also upload one of your own if you wish!
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To upload your own, hit the browse button, find the image you want to use and click 'Upload Avatar'. This will then save the image as your avatar.