You can use Code Snippets plugin if you want. If you must use functions.php, make sure it’s a child theme. If you’re placing it on a parent theme’s functions.php, it can get wiped if you update the theme. 😀
Once you get the proper function for that plugin, you can modify this line on the PHP snippet – get_avatar( get_the_author_meta( 'ID' ) ) – with the function that shows the avatar taken from the plugin.