add_action("login_head", "my_login_head");
function my_login_head() {
    echo "
	<style>
	body.login #login h1 a {
		background: url('".get_bloginfo('stylesheet_directory')."/images/logo.png') no-repeat scroll center top transparent;
		height: 120px;
		width: 450px;
		position: relative;
		left: -40px;
	}
	</style>
	";
}