capture.js

var page = require('webpage').create();
//viewportSize being the actual size of the headless browser
page.viewportSize = { width: 1024, height: 768 };
//the clipRect is the portion of the page you are taking a screenshot of
page.clipRect = { top: 0, left: 0, width: 1024, height: 768 };
//the rest of the code is the same as the previous example
page.open('http://phantomjs.org/screen-capture.html', function() {
  page.render('capture.png');
  console.log('Check capture.png');
  phantom.exit();
});