Thank you for using the Avatar Manager plugin.
Current implementation lets WordPress handle avatar uploads so they are stored in the uploads folder; an avatar image is just a regular attachment with some extra meta information (like a header or a background image). But this doesn't mean that you can't retrieve them; you can run a custom query by
avatar_manager_custom_avatar meta key for users, or
_avatar_manager_is_custom_avatar meta key for attachments.
$args = array( 'meta_key' => 'avatar_manager_custom_avatar' );
$users = get_users( $args );
foreach ( $users as $user ) ehco $user->avatar_manager_custom_avatar . PHP_EOL;
In the above example, the value of
$user->avatar_manager_custom_avatar is the attachment ID of the custom avatar image used by each user.
Let me know if this helps.