This script screenshots an array of URLs against an array of viewports after dynamically scrolling through the entire page using jQuery.

PhantomJS Screen Capture Script

'use strict';

var page = require('webpage').create();
var loadInProgress = false;
var pageIndex = 0;
var viewportIndex = 0;

var urls = [
  'https://google.com',
  'https://kokorugs.com'
];

var viewports = [
  { width: 1440, height: 805 },
  { width: 768, height: 1024 },
  { width: 320, height: 568 }
];

page.onLoadStarted = function() {
  loadInProgress = true;
  console.log('Page ' + (pageIndex + 1) + ' of ' + urls.length + ' (' + page.viewportSize.width + ' x ' + page.viewportSize.height + ') load started.');
};

page.onLoadFinished = function() {
  page.includeJs('https://ajax.googleapis.com/ajax/libs/jquery/2.2.2/jquery.min.js', function() {
    page.evaluate(function() {
      $('html, body').animate({ scrollTop: $(document).height()-$(window).height() }, 1000);
    });
    
    setTimeout(function() {
      loadInProgress = false;
      page.render('images/' + (pageIndex + 1) + '_' + page.viewportSize.width + 'x' + page.viewportSize.height + '.png');
      console.log('Page ' + (pageIndex + 1) + ' of ' + urls.length + ' (' + page.viewportSize.width + ' x ' + page.viewportSize.height + ') load finished.');
      pageIndex++;
    }, 2000);
  });
};

setInterval(function() {
  if (viewportIndex < viewports.length) {
    page.viewportSize = viewports[viewportIndex];
    if (!loadInProgress && pageIndex < urls.length) {
      page.open(urls[pageIndex]);
    }
    if (pageIndex === urls.length) {
      pageIndex = 0;
      viewportIndex++;
      console.log('Viewport ' + page.viewportSize.width + ' x ' + page.viewportSize.height + ' complete.');
    }
  } else {
    console.log('Done.');
    phantom.exit();
  }
}, 250);

console.log('Number of URLs: ' + urls.length);