screenshot.js

var page = require('webpage').create(),
    system = require('system'),
    loc, address;

if (system.args.length < 3) {
    console.log('Usage: screenshot.js <some URL> <some LOCATION>');
    phantom.exit();
}

//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 };

address = system.args[1];
loc = system.args[2];

page.open(address, function(status) {
    if (status !== 'success') {
        console.log('FAIL to load the address');
    } else {
        //save to location
        page.render(loc);
    }
    phantom.exit();
});