If you want to add your logo or the logo of your client above the wordpress login you have to put a logo.png file in the images folder of your wordpress theme and use a script like this in your functions.php file:
How to add customize logo in WordPress login
function client_wordpress_logo() { ?>
<style type="text/css">
body.login div#login h1 a {
background: transparent url(<?php echo get_stylesheet_directory_uri(); ?>/images/logo.png) top center no-repeat;
width:100%;
}
</style>
<?php }
add_action( 'login_enqueue_scripts', 'client_wordpress_logo' );