Umm..quite disagree.
First, personally I think custom avatars are a very good idea as it gives a unique identity about different users. I understand your anxiety about this and I agree with it but in that case, if someone uses a really messy or unsuitable avatar then you can always remove/edit that or ask the user to change it. But for SOME users, you shouldn't make the whole process difficult. It simply doesn't make sense (atleast to me

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While you open the Announcements board to all registered users I think still there should be a place for only approved users. Perhaps you can create another board for that.
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Keep your own dignity, both online and offline throughout your life !!!