As you know, each WordPress user account has a unique avatar. And, users usually use the Gravatar to set it up. However, it might be inconvenient in some cases. For saving time, you can completely add or change the avatar by creating a custom field that allows users to upload photos right on the website.
WordPress allows you to add more users to your website, but only from the backend. To let visitors register and manage accounts on the frontend, you can create a custom user profile page and also add custom fields for them to fill in their information. If you are using the Meta Box plugin, you can do it easily.