List of all authors avatars

I would loop through an array with the authors and then get their avatar and save them in a different array. You can use get_users() to get the users and you already know how to get their avatar.

You can get users using get_users() by giving it an array with arguments to specify what you need. You can see the available arguments to pass in the documentation for get_users(). This will return an array with users to loop through.