Screen screen = new Screen();
//set a timeout for waiting for the image
screen.setAutoWaitTimeout(30000); //default is 10 seconds
//wait for an image to get displayed on the screen and then click on it
screen.wait(new Pattern("img/image.PNG")).click();
//wait for an image with exact match
screen.wait(new Pattern("img/image.PNG").exact()).click();

import java.awt.image.BufferedImage;
 import java.awt.image.DataBuffer;
 import java.io.File;
 import java.io.IOException;
 import java.util.concurrent.TimeUnit;
 import javax.imageio.ImageIO;
 import org.apache.commons.io.FileUtils;
 import org.openqa.selenium.OutputType;
 import org.openqa.selenium.TakesScreenshot;
 import org.openqa.selenium.WebDriver;
 import org.openqa.selenium.firefox.FirefoxDriver;
 import org.testng.Assert;
 import org.testng.annotations.AfterSuite;
 import org.testng.annotations.BeforeSuite;
 import org.testng.annotations.Test;

 public class ImageComparison {

      public WebDriver driver;
      private String baseUrl;

      @BeforeSuite
      public void setUp() throws Exception {
         driver = new FirefoxDriver();
         baseUrl = "https://www.google.co.in/";
         driver.navigate().to(baseUrl);
         driver.manage().timeouts().implicitlyWait(30, TimeUnit.SECONDS);
      }

      @AfterSuite
      public void tearDown() throws Exception {
         driver.quit();
      }

      @Test
      public void testImageComparison() throws IOException, InterruptedException {
         File screenshot = ((TakesScreenshot)driver).getScreenshotAs(OutputType.FILE);
         Thread.sleep(3000);
         FileUtils.copyFile(screenshot, new File("GoogleOutput.jpg"));

         File fileInput = new File("GoogleInput.jpg");
         File fileOutPut = new File("GoogleOutput.jpg");

         BufferedImage bufferfileInput = ImageIO.read(fileInput);
         DataBuffer bufferfileInput = bufferfileInput.getData().getDataBuffer();
         int sizefileInput = bufferfileInput.getSize();
         BufferedImage bufferfileOutPut = ImageIO.read(fileOutPut);
         DataBuffer datafileOutPut = bufferfileOutPut.getData().getDataBuffer();
         int sizefileOutPut = datafileOutPut.getSize();
         Boolean matchFlag = true;
         if(sizefileInput == sizefileOutPut) {
            for(int i=0; i<sizefileInput; i++) {
                  if(bufferfileInput.getElem(i) != datafileOutPut.getElem(i)) {
                        matchFlag = false;
                        break;
                  }
             }
         }
         else {
            matchFlag = false;
         Assert.assertTrue(matchFlag, "Images are not same");
      }
 }