There are many times when, as much as I’d love to publicize that a site is built on WordPress, the logo just isn’t appropriate on the login screen (wp-login.php). Thankfully, it’s quite easy to replace the logo using a bit of CSS. Simply paste the following snippet of code into your theme’s functions.php file and adjust the CSS on line 4 as needed.
//Theme login screen
function replace_login_logo() {
$style = '<style type="text/css">';
$style .= '#login h1 { background: url( ' . get_stylesheet_directory_uri() . '/images/logo.gif ) no-repeat scroll center top transparent; display: block; height: 118px; overflow: hidden; padding-bottom: 15px; text-indent: -9999px; width: 369px; margin-left: -25px; }';
$style .= '#login h1 a { display: none; }';
$style .= '</style>';
echo $style;
}
add_action( 'login_head', 'replace_login_logo' );