Add custom logo in /wp-admin

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