Paste the following in functions.php:
function my_login_logo() { ?> <style type="text/css"> #login h1 a, .login h1 a { background-image: url(http://linktolog.com); height:65px; width:320px; background-size: 250px 67px; background-repeat: no-repeat; padding-bottom: 0px; } </style> <?php } add_action( 'login_enqueue_scripts', 'my_login_logo' );
Change parameters as needed.
Source: https://codex.wordpress.org/Customizing_the_Login_Form#Styling_Your_Login