后台文件:

 1using System;
 2using System.Data;
 3using System.Configuration;
 4using System.Collections;
 5using System.Drawing;
 6using System.Web;
 7using System.Web.Security;
 8using System.Web.UI;
 9using System.Web.UI.WebControls;
10using System.Web.UI.WebControls.WebParts;
11using System.Web.UI.HtmlControls;
12
13public partial class Default2 : System.Web.UI.Page
14{
15    PRotected void Page_Load(object sender, EventArgs e)
16    {
17        if(!this.IsPostBack)
18        {
19            this.GenImg(this.GenCode(4));
20        }
21
22    }
23    //产生随机字符串
24    private string GenCode(int num)
25    {
26        string[] source={"0","1","2","3","4","5","6","7","8","9",
27                            "A","B","C","D","E","F","G","H","I","J","K","L","M","N",
28                            "O","P","Q","R","S","T","U","V","W","X","Y","Z"};
29        string code="";
30        Random rd=new Random();
31        for(int i=0;i < num;i++)
32        {
33            code += source[rd.Next(0,source.Length)];
34        }
35        return code;
36    }
37
38        //生成图片
39    private void GenImg(string code)
40    {
41        Bitmap myPalette = new Bitmap(60, 20);//定义一个画板
42
43        Graphics gh = Graphics.FromImage(myPalette);//在画板上定义绘图的实例
44
45        Rectangle rc = new Rectangle(0, 0, 60, 20);//定义一个矩形
46
47        gh.FillRectangle(new SolidBrush(Color.Blue), rc);//填充矩形
48        gh.DrawString(code, new Font("宋体", 16), new SolidBrush(Color.White), rc);//在矩形内画出字符串
49
50        myPalette.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);//将图片显示出来
51
52        session["ValidateCode"] = code;//将字符串保存到Session中，以便需要时进行验证
53
54        gh.Dispose();
55        myPalette.Dispose();
56    }
57
58}
59HTML文件:
加入一个HTML控件Image即可
1<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default2.aspx.cs" Inherits="Default2" %>
2
3<img src="Default2.aspx" />