import java.awt.Dimension;
import java.awt.Rectangle;
import java.awt.Robot;
import java.awt.Toolkit;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;
import java.io.File;

...

public void captureScreen(String fileName) throws Exception {

   Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
   Rectangle screenRectangle = new Rectangle(screenSize);
   Robot robot = new Robot();
   BufferedImage image = robot.createScreenCapture(screenRectangle);
   ImageIO.write(image, "png", new File(fileName));
}
...

var page = require('webpage').create();

page.settings.userAgent = 'UltimateBrowser/100';
page.viewportSize = { width: 1200, height: 1200 };
page.clipRect = { top: 0, left: 0, width: 1200, height: 1200 };

page.open('https://google.com/', function () {
  page.render('output.png');
  phantom.exit();
});

<script type="text/javascript"
src="http://grabz.it/services/javascript.ashx?
key=APPLICATION KEY&url=http://www.google.com">
</script>

var element = document.createElement("FireShotDataElement");
element.setAttribute("Entire", true);
element.setAttribute("Action", 1);
element.setAttribute("Key", "");
element.setAttribute("BASE64Content", "");
element.setAttribute("Data", "C:/Users/jagilber/Downloads/whatev.jpg");

if (typeof(CapturedFrameId) != "undefined")
        element.setAttribute("CapturedFrameId", CapturedFrameId);


document.documentElement.appendChild(element);

var evt = document.createEvent("Events");
evt.initEvent("capturePageEvt", true, false);

element.dispatchEvent(evt);