[Plugin: Author Avatars List] Using username in shorcode

I had a specific need to show author avatars using username (loginname) rather than email or userid. Since I could not find anything, I wrote my first plugin. I am not a programmer and the code below is probably not without problems – but I post it here in the hope that it may help someone.
1. Copy the code below and save it as a php file.
2. Upload the file to your plugins folder and activate it.
3. Where you want the avatar to appear, use [showavatar username=loginname size=number] in the post.

<?php
/*
Plugin Name: Show User Avatar
Description: A plugin designed to provide shortcode ability in posts and comments using showavatar shortcode. Valid attributes are username=loginname  and size=number.
Author: sargespam
Version: 0.8
*/

add_shortcode('showavatar', 'getuseravatar');
function getuseravatar($atts, $content=null) {
	    extract(shortcode_atts(array(
		'username' => '',
		'size' => '',
		), $atts)) ;

		$user = get_userdatabylogin($username);
		$useridnum = $user->ID;

	$output = get_avatar($useridnum, $size) ;
	return $output;
}

?>

Alternately, I believe you can also use the following code in your theme’s functions.php file rather than making it a plugin.

function getuseravatar($atts, $content=null) {
	    extract(shortcode_atts(array(
		'username' => '',
		'size' => '',
		), $atts)) ;

		$user = get_userdatabylogin($username);
		$useridnum = $user->ID;

	$output = get_avatar($useridnum, $size) ;
	return $output;
add_shortcode('showavatar', 'getuseravatar');

https://www.ads-software.com/extend/plugins/author-avatars/