Screenshot screen by index. Crop function is a free bonus.

screenshotter.cs

using System.Windows.Forms;
using System.Drawing;
using System.Drawing.Imaging;

class Program
{
    static Bitmap screenshot;
    static Graphics gfxScreenshot;

    [STAThreadAttribute]
    static void Main(string[] args)
    {
        while (true)
        {

            Console.WriteLine("Press enter to take shot");
            Console.ReadLine();
            Screen screen = Screen.AllScreens[1];
            screenshot = new Bitmap(screen.Bounds.Width,
                screen.Bounds.Height,
                System.Drawing.Imaging.PixelFormat.Format32bppArgb);
            // Create a graphics object from the bitmap
            gfxScreenshot = Graphics.FromImage(screenshot);
            // Take the screenshot from the upper left corner to the right bottom corner
            gfxScreenshot.CopyFromScreen(
                screen.Bounds.X,
                screen.Bounds.Y,
                0,
                0,
                screen.Bounds.Size,
                CopyPixelOperation.SourceCopy);
            string savePath = String.Format(@"C:\test.jpeg");
            screenshot.Save(savePath, ImageFormat.Jpeg);
        }
    }

    static Image CropImage(Image toBeCropped, RectangleF cropBounds)
    {
        Bitmap b = toBeCropped as Bitmap;
        Bitmap target = new Bitmap( Convert.ToInt16(cropBounds.Width), Convert.ToInt16(cropBounds.Height));
        using (Graphics g = Graphics.FromImage(target))
        {
            g.DrawImage(b, new RectangleF(0, 0, target.Width, target.Height), cropBounds, GraphicsUnit.Pixel);
        }

        return target;
    }
}