how to download image in asp.net

 using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Diagnostics;
using System.Web.UI.WebControls;
using System.Data;
using System.IO;


protected void imu_Click(object sender, ImageClickEventArgs e)
    {
        ImageButton imb = (ImageButton)sender;
        string s = Server.MapPath("image//") + imb.CommandArgument.ToString();
        string fName = @s.ToString();
        FileInfo fi = new FileInfo(fName);
        long sz = fi.Length;
        Response.ClearContent();
        Response.ContentType = MimeType(Path.GetExtension(fName));
        Response.AddHeader("Content-Disposition", string.Format("attachment; filename = {0}", System.IO.Path.GetFileName(fName)));
        Response.AddHeader("Content-Length", sz.ToString("F0"));
        Response.TransmitFile(fName);
        Response.End();
    }
    public static string MimeType(string Extension)
    {
        string mime = "application/octetstream";
        if (string.IsNullOrEmpty(Extension))
            return mime;
        string ext = Extension.ToLower();
        Microsoft.Win32.RegistryKey rk = Microsoft.Win32.Registry.ClassesRoot.OpenSubKey(ext);
        if (rk != null && rk.GetValue("Content Type") != null)
            mime = rk.GetValue("Content Type").ToString();
        return mime;
    }