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Go to the admin control panel, click on preferences, click on edit by User Info Settings, The 11 line down put a tick mark in yes by "Allow custom avatar upload?" Next line down you can set restriction by number of post for custom avitar. Save your changes.
Go to view account, click on the avitar button, you will now have a browse button to upload your custom avitar. I believe default maximum size is 120x120 pixels.
HTH.