screenshot.js

var casper = require("casper").create(),
    viewportSizes = [
      [320,480],
      [320,568],
      [600,1024],
      [1024,768],
      [1280,800],
      [1440,900]
    ],
    url = casper.cli.args[0],
    saveDir = url.replace(/[^a-zA-Z0-9]/gi, '-').replace(/^https?-+/, '');

casper.start();

casper.each(viewportSizes, function(self, viewportSize, i) {
  var width = viewportSize[0],
      height = viewportSize[1];

  casper.wait(5000, function() {
    this.viewport(width, height);
    casper.thenOpen(url, function() {
      this.echo('Opening at ' + width);
      var FPfilename = saveDir + '/fullpage-' + width + ".png";
      var ACfilename = saveDir + '/' + width + '-' + height + ".png";
      this.captureSelector(FPfilename, 'body');
      this.capture(ACfilename,{top: 0,left: 0,width: width, height: height});
      this.echo('snapshot taken');
    });
  });
});

casper.run(function() {
  this.echo('Finished captures for ' + url).exit();
});