using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;

public partial class image : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        string tmp = RanNum(4);
        HttpCookie a = new HttpCookie("ImageV",tmp);
        Response.Cookies.Add(a);
        this.ValidateCode(tmp);
    }
    public void ValidateCode(string VNum)
    {
        Bitmap Img = null;
        Graphics g = null;
        MemoryStream ms = null;

        int gheight = VNum.Length * 2;
        Img = new Bitmap(gheight,25);
        g = Graphics.FromImage(Img);
        //设定背景颜色
        g.Clear(Color.White);
        //文字字体
        Font f=new Font ("Arial Black",10);
        //文字颜色
        SolidBrush s = new SolidBrush(Color.Black);
        g.DrawString(VNum,f,s,3,3);
        ms = new MemoryStream();
        Img.Save(ms,ImageFormat.Jpeg);
        
        Response.ClearContent();
        Response.ContentType = "image/jpeg";
        Response.BinaryWrite(ms.ToArray());

        g.Dispose();
        Img.Dispose();
        Response.End();
    }
    private string RanNum(int VCodeNum)
    {
        string Vchar = "0,1,2,3,4,5,6,7,8,9,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,"
                        + "q,r,s,t,u,v,w,x,y,z";
        string[] VcArray = Vchar.Split(new char []{','});
        string VNum = "";
        int Temp = -1;

        Random rand = new Random();

        for (int i = 1; i < VCodeNum + 1; i++)
        {
            if (Temp != -1)
            { rand = new Random(i * Temp * unchecked((int)DateTime.Now.Ticks)); }
            int t = rand.Next(35);
            if (Temp != -1 && Temp == t)
            { return RanNum(VCodeNum); }
            Temp = t;
            VNum += VcArray[t];
        }
        return VNum;
    }
}

<asp:ImageButton ID="ImageButton1" runat="server" />

public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {
        ImageButton1.ImageUrl = "image.aspx";
    }
}