Is it possible (as per each users choice) to show avatars to registered members only ?
Literally so that a member can choose whether he wants to have his avatar shown publicly or whether he wants to show his avatar to other registered members only.
Then you can reference that field in the templates to create conditionals. The problem is that avatars are used in many templates. And more than that, custom fields aren't available everywhere. So this is more than a simple hack.