WordPress - Change login logo

functions.php

<?php
add_action( 'login_head', 'custom_login_logo' );
function custom_login_logo() {
	echo '
		<style type="text/css">
			h1 a { background-image: url('.get_template_directory_uri().'/img/new-logo.png) !important; }
		</style>
	';
}