To add a custom class in wordpress avatar image, we have to use filter hooks in get_avatar function.

Filter hooks example:

//avatar custom class

function my_avatar_custom_class($avatar, $id_or_email, $size, $default, $alt, $args) { 

	$avatar = '<img src='. get_avatar_url( get_the_author_meta('ID'), $size = '80' ) .' class=" avatar avatar-' . $size .' photo rounded-circle" height='. $size .' width='. $size .' />';

	return $avatar;
}

// add the filter
add_filter( "get_avatar", "my_avatar_custom_class", 10, 6 );

get_avatar function for details : https://developer.wordpress.org/reference/functions/get_avatar/

appy filter hooks for details : https://developer.wordpress.org/reference/hooks/get_avatar/