<?php
function mi_logo_login_personalizado() {
    ?>
    <style type="text/css">
        #login h1 a, .login h1 a {
            background-image: url(<?php echo get_stylesheet_directory_uri(); ?>/img/mi-logo.png);
            height: 65px;
            width: 320px;
            background-size: contain;
            background-repeat: no-repeat;
            padding-bottom: 30px;
        }
    </style>
    <?php
}
add_action( 'login_enqueue_scripts', 'mi_logo_login_personalizado' );