How to divide image into sub blocks 8x8 in c#?

I need to make Image Watermarking in DCT method using c#. But I don't know how to dividing image into 8x8 blocks and make the DCT transform using c#. Anyone can help me please? So far this is what I've tried.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

using Emgu.CV;
using Emgu.CV.Util;
using Emgu.CV.Structure;

namespace DCTTransform
{
    public partial class Form1 : Form
    {
        Image<Bgr, Byte> img;
        Image<Ycc, Byte> imgYcc;
        Bitmap bmp;


        public Form1()
        {
            InitializeComponent();
        }

        private void btBrowse_Click(object sender, EventArgs e)
        {
            OpenFileDialog od = new OpenFileDialog();
            if (od.ShowDialog() == DialogResult.OK)
            {
                img = new Image<Bgr, byte>(od.FileName);
                pcCitra.Image = img.ToBitmap();
            }
        }



        private void btTransform_Click(object sender, EventArgs e)
        {
            imgYcc = img.Convert<Ycc, Byte>();
            bmp = imgYcc[0].ToBitmap();
            pbDCT.Image = bmp;

        }
    }
}

Thanks before :)