To alter you avatar, simply go to your user page and roll your mouse over the avatar picture. If you've not yet chosen an avatar, it will look like a grey, featureless person. When your mouse hits the avatar, a hidden "edit" button will suddenly appear. If you click on the edit button, you will then be presented with a row of pre-set avatars, as well as a button which says "choose". Now you will have a new avatar, which will be visible regardless of the wiki you're visiting at Wikia.