Customizing the Login Form « WordPress Codex

//login logo
function my_login_logo() { ?>
    <style type="text/css">
        body.login div#login h1 a {
            background-image: url(<?php echo get_stylesheet_directory_uri(); ?>/images/site-login-logo.png);
            padding-bottom: 30px;
            width: 300px;
            background-size: auto auto;
        }
    </style>
<?php }
add_action( 'login_enqueue_scripts', 'my_login_logo' );

via Customizing the Login Form « WordPress Codex.