They have this policy in place for 1 simple reason: Time. By allowing custom (user uploadable) avatars they have to review and approve every single one to ensure it is not obscene or otherwise offensive. With millions of users, if they allowed multiple changes they would be spending an incredible amount of time reviewing them.

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If you feel you have a question, dont be lazy; make sure you have asked yourself first. Then search the forum, then just fucking google it to get as much detail as you can. You may then seek clarification on specific points from others.