Avatar can be used to show user's profile picture on profile information page, on navigation bar, in blogs grid items.
Avatars are available in 4 sizes avatar-lg, avatar-md, avatar-sm & avatar-ex-sm. to add avatar you have to include avatar class, for size add class sizes according to sizes.
This avatar stacks are using avatar-sm classes. to stack the avatar use avatar-group for every images and wrap those images inside avatar class
Text avatar can be used for to add initial letters for user. Inside avatar-hw-lg, h represent height and w represend width