Change the Login Page Logo and Link in WordPress

The following code lets you change the logo on the WordPress login page as well as the link it points to:

add_filter( 'login_headerurl', 'namespace_login_headerurl' );
function namespace_login_headerurl( $url ) {
    $url = home_url( '/' );
    return $url;
}

function namespace_login_headertitle( $title ) {
    $title = get_bloginfo( 'name' );
    return $title;
}
add_filter( 'login_headertitle', 'namespace_login_headertitle' );

function namespace_login_style() {
    echo '<style>.login h1 a { background-image: url( ' . get_template_directory_uri() . '/images/logo.png ) !important; }</style>';
}
add_action( 'login_head', 'namespace_login_style' );

For more login page security tips, see Hardening WordPress Security.