If you want to change the logo, you could hook into login_head
and output css that replaces the default logo. This goes into your functions.php.
function my_custom_login_logo() {
// as below or link to an external .css-file
echo '<style type="text/css">
div#login h1 a {
background-image:url('urltoyourlogo.png')
}
</style>';
}
add_action('login_head', 'my_custom_login_logo');