Modificar el logo de la pantalla de inicio de sesión

WordPress 5.2 incorporó un nuevo filtro que nos facilita esta tarea.

<?php
/**
 * Custom login form logo link title.
 * 
 * Requires WordPress v5.2+ in order to use the login_headertext filter
 */
function acme_login_logo_image( $login_header_text ) {
 
    $logo_url          = 'YOUR_IMAGE_URL_HERE';
 
    $login_header_text = ''; // clears default output.
    $login_header_text = '<img src="' . $logo_url . '" alt="' . get_bloginfo( 'title' ) . '" />';
 
    return $login_header_text;
 
}
 
// Add filter if WP version is 5.2+
if ( 5.2 >= get_bloginfo( 'version' ) ) {
    add_filter( 'login_headertext', 'acme_login_logo_image' );
}