Principal.cs

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

namespace R806CapturaPantalla
{
    public partial class Principal : Form
    {
        public Principal()
        {
            InitializeComponent();
        }

        private void btnCapturarPantalla_Click(object sender, EventArgs e)
        {
            Bitmap bmpCapturaPantalla = new Bitmap(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height);

            using (Graphics g = Graphics.FromImage(bmpCapturaPantalla))
            {
                g.CopyFromScreen(0, 0, 0, 0, bmpCapturaPantalla.Size);
            }

            pbxCapturaPantalla.Image = bmpCapturaPantalla;
        }
    }
}