When you want to take screenshots with PhantomJS, you probably need to wait before your page is completely loaded. I mean all resources are loaded and scripts executed.

Use the following code to delay your screenshot :

var page = new WebPage();
page.open('http://stackoverflow.com/', function (status) {
        just_wait();
});

function just_wait() {
    setTimeout(function() {
            page.render('screenshot.png');
            phantom.exit();
    }, 2000);
}

Here we wait 2000ms before saving the screenshot. Adjust it to fit your needs.

Edit 2016-08-31

Since phantomjs 1.2, you can use the callback onLoadFinished() which is much better than a simple guess on the rendering time…

var page = new WebPage();
page.open('http://charles.lescampeurs.org/');
page.onLoadFinished = function(status) {
    page.render('screenshot.png');
    phantom.exit();
};