I like to change the login image for all the websites I develop in WordPress so that the site is more consistent and users are logging in to their own site, not WordPress. To do this, I take a 2 step approach.

Upload the logo you want into the images folder within the theme folder, example:

http://domainname.com/wp-content/themes/default/images/logo-login.gif

Then add the following code to your functions.php file in the theme:

<pre><code>add_action("login_head", "my_login_head");
function my_login_head() {
	echo "
	<style>
	body.login #login h1 a {
		background: url('".get_bloginfo('template_url')."/images/logo-login.gif') no-repeat scroll center top transparent;
		height: 90px;
		width: 400px;
	}
	</style>
	";
}