实现：
添加命名空间

using System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D;
生成页代码

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D;

public partial class getRandImg : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        //生成随机码图片
        SetValidateCode();
        //生成页面不保存到cache
        Response.Cache.SetNoStore();
    }

    //设置验证码
    private void SetValidateCode()
    {
        //新建位图
        Bitmap newBitmap = new Bitmap(
                                        71,
                                        23,
                                        PixelFormat.Format32bppArgb
                                     );
        //从位图获得绘图画面
        Graphics g = Graphics.FromImage(newBitmap);
        //随机数生成器
        Random r = new Random();
        //绘图画面清空
        g.Clear(Color.White);
        //绘图画面划线干扰
        for (int i = 0; i < 50; i++)
        {
            int x1 = r.Next(newBitmap.Width);
            int x2 = r.Next(newBitmap.Width);
            int y1 = r.Next(newBitmap.Height);
            int y2 = r.Next(newBitmap.Height);
            g.DrawLine(new Pen(
                                Color.FromArgb(r.Next())),
                                x1,
                                y1,
                                x2,
                                y2
                              );
        }
        //绘图画面点数干扰
        for (int i = 0; i < 100; i++)
        {
            int x = r.Next(newBitmap.Width);
            int y = r.Next(newBitmap.Height);
            newBitmap.SetPixel(
                                x,
                                y,
                                Color.FromArgb(r.Next())
                              );
        }
        //获得随机字符串（5位长度）
        string value = GenerateRandom(5);
        //随机字符串赋值给Session
        Session["RandCode"] = value;
        //定义图片显示字体样式
        Font font = new Font(
                               "Arial",
                               14,
                               FontStyle.Bold
                            );
        Random rr = new Random();
        int yy = rr.Next(1, 4);
        //定义随机字符串显示图片刷子
        LinearGradientBrush brush = new LinearGradientBrush(
                                                              new Rectangle(0, 0, 71, 23),
                                                              Color.Red,
                                                              Color.Blue,
                                                              1.2f,
                                                              true
                                                           );
        g.DrawString(value, font, brush, 2, yy);
        g.DrawRectangle(new Pen(
                                  Color.Silver),
                                  0,
                                  0,
                                  70,
                                  22
                                );
        System.IO.MemoryStream ms = new System.IO.MemoryStream();
        newBitmap.Save(ms, ImageFormat.Gif);
        //输出图片
        Response.ClearContent();
        Response.ContentType = "image/gif";
        Response.BinaryWrite(ms.ToArray());
    }

    //常量集
    private static char[] constant ={
                                        '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'
                                    };

    //生成随机字符串
    public static string GenerateRandom(int Length)
    {
        System.Text.StringBuilder newRandom = new System.Text.StringBuilder(36);
        Random rd = new Random();
        for (int i = 0; i < Length; i++)
        {
            newRandom.Append(constant[rd.Next(36)]);
        }
        return newRandom.ToString();
    }
}

使用随机图片的页面，IMAGE控件的写法如下：

<asp:Image ID="Image1" ImageUrl="~/getRandImg.aspx" runat="server" />

示例代码：