/* Change logo on login page */
function my_login_logo() { ?>
    <style type="text/css">
        #login h1 a, .login h1 a {
            background-image: url(<?php echo get_stylesheet_directory_uri(); ?>/путь/к/картинке.png);
		height:XXXpx;
		width:XXXpx;
		background-size: XXXpx XXXpx;
		background-repeat: no-repeat;
		margin-top: XXXpx;
        	padding-bottom: XXXpx;
        }
    </style>
<?php }
add_action( 'login_enqueue_scripts', 'my_login_logo' );