一句代码的事,就不多说了

[php]
function custom_login_head(){
$str=file_get_contents('http://cn.bing.com/HPImageArchive.aspx?idx=0&n=1');
if(preg_match("/<url>(.+?)<\/url>/ies",$str,$matches)){
$imgurl='http://cn.bing.com'.$matches[1];
echo'<style type="text/css">body{background: url('.$imgurl.');width:100%;height:100%;background-image:url('.$imgurl.');-moz-background-size: 100% 100%;-o-background-size: 100% 100%;-webkit-background-size: 100% 100%;background-size: 100% 100%;-moz-border-image: url('.$imgurl.') 0;background-repeat:no-repeat\9;background-image:none\9;}h1 a { background-image:url('.get_bloginfo('url').'/favicon.ico)!important;width:32px;height:32px;-webkit-border-radius:50px;-moz-border-radius:50px;border-radius:50px;}</style>';
}}
add_action('login_head', 'custom_login_head');
[/php]