Bitmap.HorizontalResolution

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

public class Form1 : Form {

    protected override void OnPaint(PaintEventArgs e) {
    Graphics g = e.Graphics;
    Bitmap bmp = new Bitmap("rama.jpg");
    g.DrawImage(bmp, 0, 0);

    Console.WriteLine("Screen resolution: " + g.DpiX + "DPI");
    Console.WriteLine("Image resolution: " + bmp.HorizontalResolution + "DPI");
    Console.WriteLine("Image Width: " + bmp.Width);
    Console.WriteLine("Image Height: " + bmp.Height);

    SizeF s = new SizeF(bmp.Width * (g.DpiX / bmp.HorizontalResolution),
              bmp.Height * (g.DpiY / bmp.VerticalResolution));
    Console.WriteLine("Display size of image: " + s);
    }
    public static void Main() {
        Application.Run(new Form1());
    }
}