phantomjs

gistfile1.js

var args = require('system').args;
var webpage = require('webpage');
var page = webpage.create();
var url = args[1];
page.viewportSize = {
            width: 1024,
            height: 768
        };
page.open(url, function () {
    console.log(page.renderBase64('PNG'));
    phantom.exit();
});