Don’t just change your login logo, change your admin logo as well. Make sure you add your admin_logo.png to your images folder.

 
function custom_admin_logo() { 
echo '<style type="text/css"> 
#header-logo { background-image: url('.get_bloginfo('template_directory').'/images/admin_logo.png) !important; } 
</style>'; 
} 
add_action('admin_head', 'custom_admin_logo');