// CUSTOM ADMIN LOGIN HEADER LOGO  
  
function my_custom_login_logo()  
{  
    echo '<style  type="text/css"> h1 a {  background-image:url('.get_bloginfo('template_directory').'/lib/admin/images/logo_admin.png)  !important; } </style>';  
}  
add_action('login_head',  'my_custom_login_logo');

http://wp.tutsplus.com/tutorials/customizing-wordpress-for-your-clients/

When a client wants to access the WordPress admin, the first thing they will notice is the login page with a WordPress logo. Wouldn’t it be better if this page had the logo of your client’s company or maybe yours? There are many simpler ways to do this using plugins that allow other customization options too. But I personally prefer setting this using the powerful WordPress functions.php file. 
If you have a functions.php in your custom theme, open it and if not, create a file named funtions.php and place it inside the theme folder.
Add below lines in the functions.php file:

Custom Admin Login Header Logo

header, wordpress

PHP