A custom WordPress Login Logo without a Plugin

functions.php

<?php
function my_custom_login_logo() {
    echo '<style type="text/css">
        h1 a { background-image:url('.get_bloginfo('template_directory').'/images/custom-login-logo.gif) !important; }
    </style>';
}

add_action('login_head', 'my_custom_login_logo');
?>