use nodejs to download bing's wallpaper

i want to download bing's todays wallpaper by node.js,

first of all,i can download large pictures from some other sites,

but i can't download the pic from bing,even though i can see the pic in my browser

the example picture src is : "http://www.bing.com/az/hprichbg?p=rb%2fYosemiteSnow_EN-US7191433727_1920x1200.jpg"

here's my core code

var download_file_httpget = function(file_url) {
    var options = {
        host: url.parse(file_url).host,
        port: 80,
        path: url.parse(file_url).pathname,
        headers: {}
    };
    var file_name = '11.jpg';
    var file = fs.createWriteStream(download_path + "\\" + file_name);
    http.get(options,function(res) {
        res.on('data',function(data) {
            file.write(data);
        }).on('end',function() {
            file.end();
            console.log('download success');
        });
    });
};

for example,the orginal pic's size is 200k,but it only download 20k,and i doubt that it is because of the 'referer' in the header,when i add the header,the problem fixed in some oher sites,but it still dosen't work for bing