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As far as I know,on iPod version(therefore the mobile version) you can't change avatar.
To change it,you must press the button "Full Version" on the
bottom of the screen to change to the PC version.
When you change to the normal version,enter on your profile,and click on the "Change" button that will be next to your avatar.
As a normal user,you have a limit of 150x150 pixels,and up to 255kb
Every page has a limit of 50 notifications,divided on 10 pages.The conplete notification system keeps up to 500 notifications,divided through those 10 pages.
Lightwing's post in Changing the "nickname" of sorts was marked as the answer