How to draw multiple rectangles on an image according to the size of an image automatically

I'm working on a project , where i need to create user friendly rectangles on top of the image , i'm able to draw rectangles now on top of the image , but i'm not getting about how to calculate required rectangles for a image , Image size may vary , for example if an image size is 675*1200 , i want to draw 48 rectangles on it and if i alter one rectangle , it should alter all the other rectangles .

currently i'm getting multiple rectangles on image and re sizable, but i want those in order and they alter the dimensions at a single time .

        int j = pictureBox1.Location.X;
        int k = pictureBox1.Location.Y;

        int imagewidth = pictureBox1.Image.Size.Width;
        int imageheight = pictureBox1.Image.Size.Height;

        System.Drawing.Image img = System.Drawing.Image.FromFile(@"C:li.jpg");
        MessageBox.Show("Width: " + img.Width + ", Height: " + img.Height);

        var size = pictureBox1.ClientSize;
       // private bool _once = true; 600*400 = 2400  , X=12,Y=12 . 
        for ( k = 13; k <= 2400; k++)
        {
            k = k - 1;
            //Userrect class
            rect = new UserRect(new Rectangle(j, k, 266, 266));
            // Added to list
            listRec.Add(rect.rect);
            rect.SetPictureBox(this.pictureBox1);
            //i = i + 20;
            //j = j + 12;
            k = k + 266;

        }