WordPress Admin Login Custom Logo

I personally like to use something like this. This will look for a logo image in your templates /images/ folder or directory, then display it in place of the default WordPress Logo.

function my_login_logo() { 
    if(file_exists(TEMPLATEPATH.'/images/logo.png'))
        $logo = '/images/logo.png';
    else if(file_exists(TEMPLATEPATH.'/images/logo.jpg'))
        $logo = '/images/logo.jpg';
    else if(file_exists(TEMPLATEPATH.'/images/logo.gif'))
        $logo = '/images/logo.gif';
?>
    <style type="text/css">
        body.login div#login h1 a {
            background-image: url(<?php echo get_bloginfo('template_directory').$logo; ?>);
            padding-bottom: 40px;
            background-size: auto;
            width: auto;
            height: auto;
        }
    </style>
<?php 
}
add_action( 'login_enqueue_scripts', 'my_login_logo' );

Alternatively as @G.M. suggested in the comments, you could use a more child_theme friendly approach like so:

function my_login_logo() { 
    $locate = locate_template( array('images/logo.png', 'images/logo.jpg', 'images/logo.gif'), false ); 
    if ( empty($locate) ) return;
    $base = is_child_theme() && substr_count($locate , get_stylesheet_directory()) ? get_stylesheet_directory_uri() : get_template_directory_uri();
    $logo = $base . '/images/' . basename($locate);
?>
<style type="text/css">
body.login div#login h1 a {
  background-image: url(<?php echo $logo; ?>);
  padding-bottom: 40px;
  background-size: auto;
  width: auto;
  height: auto;
}
</style>
<?php 
}
add_action( 'login_enqueue_scripts', 'my_login_logo' );

 

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The following 2 functions will change where you get directed to once you click the logo banner, which in my first function I replace it with a link to the homepage.

The 2nd function will replace the hover text to the blog descriptoin that was set in Settings as your Tag Line.

/** Change Banner Link **/
function custom_loginlogo_url($url) {
    return home_url();
}
add_filter( 'login_headerurl', 'custom_loginlogo_url' );

/** Change Link Title to Tagline **/
function custom_login_logo_link_title(){
    return get_option('blogdescription');  
}
add_filter('login_headertitle', 'custom_login_logo_link_title');

All these need to be placed into function.php for them to work.