var recHeight = Convert.ToInt32(img.Height * 0.23);
var recWidth = Convert.ToInt32(img.Width * 0.33);

var posH = Convert.ToInt32((img.Height - recHeight) / 2);
var posW = Convert.ToInt32((img.Width - recWidth) / 2);

Rectangle rec = new Rectangle(posH, posW, recWidth, recHeight);

Bitmap bmp = new Bitmap(rec.Width, rec.Height);
using (Graphics gph = Graphics.FromImage(img))
    {
        Pen pen = new Pen(Color.Red);
        gph.DrawRectangle(pen, rec);
     }