function custom_loginlogo() {
echo '<style type="text/css">
h1 a {background-image: url('.get_bloginfo('template_directory').'/images/login_logo.png) !important; }
</style>';
}
add_action('login_head', 'custom_loginlogo');





source: http://www.wpbeginner.com/wp-tutorials/how-to-change-the-login-logo-in-wordpress/#