AdvancedWebDriverUsageTests.cs

public void TakeScreenshotOfElement(IWebDriver driver, By by, string fileName)
{
    // 1. Make screenshot of all screen
    var screenshotDriver = driver as ITakesScreenshot;
    Screenshot screenshot = screenshotDriver.GetScreenshot();
    var bmpScreen = new Bitmap(new MemoryStream(screenshot.AsByteArray));

    // 2. Get screenshot of specific element
    IWebElement element = driver.FindElement(by);
    var cropArea = new Rectangle(element.Location, element.Size);
    var bitmap = bmpScreen.Clone(cropArea, bmpScreen.PixelFormat);
    bitmap.Save(fileName);
}