Co-Authors Plus: How do I get all authors with a query?

If you are using Co-Authors Plus plugin. By default, it will show the original author of the post.

You’ll need to edit theme files with the code responsible for showing the author name for posts. This could be single.php, content.php, or a template tag in your theme’s functions.php file.

the_author_posts_link() and you will need to replace it with the following code snippet.

if ( function_exists( 'coauthors_posts_links' ) ) {
    coauthors_posts_links();
} else {
    the_author_posts_link();
}

Reference

You can use the following code to get co-authors.

<?php $coauthors = get_coauthors(); ?>
<?php foreach( $coauthors as $coauthor ): ?>
<center><div class="authbio">
<img src="https://wordpress.stackexchange.com/questions/318955/<?php bloginfo("template_url'); ?>/images/<?php echo $coauthor->user_firstname; ?>.jpg" alt="" class="alignleft"/>
<?php $userdata = get_userdata( $coauthor->ID ); ?>
<?php if ( $userdata->user_description ) echo $userdata->user_description; ?>
</div></center>
<?php endforeach; ?>