Changing user avatars
When a new user registers on a site, the system assigns a default avatar to them. After that, they can change their avatar on a page containing the My account web part. This can be done on the Personal settings tab.
Here users can delete their avatar, upload a new one, or select a predefined avatar.
You can find a live example of this on the Community starter site. Log-in as one of the predefined users (e.g. Turbo with blank password) and click the Edit my profile link in the Shortcuts menu on the right.
Changing user avatars in Administration
Users with access to the administration interface can change their avatars in the My profile application.