There are many ways to capture and name the image of the screen. Using the test method name and a timestamp for the image name is a common practice. This aligns the captured screens with the test methods that created them, putting a date on the filename, and so on:

/**
 * screenShot - method that takes iTestResult as parameter
 * 
 * @param result - The result of test
 * @return String
 */
public static String screenShot(ITestResult result) throws Exception {
    DateFormat stamp = new SimpleDateFormat("MM.dd.yy.HH.mm.ss");
    Date date = new Date();

    ITestNGMethod method= result.getMethod();
    String testName = method.getMethodName();

    return captureScreen(testName + "_" + stamp.format(date) + ".png");
}

/**
 * captureScreen - method to capture the entire screen of the Browser
 * or Mobile App
 *
 * @param filename - The filename to save it to
 */
 public static String captureScreen(String filename) throws Exception {
    String bitmapPath = "myPath";
    WebDriver driver = CreateDriver.getInstance().getCurrentDriver();
    File screen = null;

    if ( Global_VARS.DEF_ENVIRONMENT.equalsIgnoreCase("remote") ) {
        // cast to Augmenter class for RemoteWebDriver
        screen = ((TakesScreenshot)new Augmenter().augment(driver))
                 .getScreenshotAs(OutputType.FILE);
    }

    else {
        screen = ((TakesScreenshot)driver)
                 .getScreenshotAs(OutputType.FILE);
    }

    FileUtils.copyFile(screen, new File(bitmapPath + filename));
    return filename;
}