If you want to change the default wordpress logo on the login page, you can add the following code to your themes functions.php file. Make sure /images/login-logo.png is present in your theme folder.
function custom_login_logo() {
echo '<style type="text/css">h1 a { background-image:url('.get_bloginfo('template_url').'/images/login-logo.png) !important; }</style>';
}
add_action('login_head', 'custom_login_logo');
tested with wordpress version 3.3