我不能把验证码写入图片中。。怎么办！

using System;
using System.Data;
using System.Configuration;
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;
public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        string str = suiji(4);
        Bitmap bit = new Bitmap(50, 20);
        Graphics g = Graphics.FromImage(bit);
        Font text = new Font("宋体",20);
        Rectangle rec = new Rectangle(0,0,50,20);
        g.FillRectangle(new SolidBrush(Color.Green), rec);      
        g.DrawString(str, text, new SolidBrush(Color.Blue),rec);
        bit.Save("yanzhen.jpg",ImageFormat.Jpeg);
        Image1.ImageUrl = "yanzhen.jpg";
    }
    public string suiji(int a)
    {
        string result = "";
        string str = "0,1,2,3,4,5,6,7,8,9";
        string[] arry = str.Split(',');
        Random ram = new Random();
        
        for (int i = 0; i < a; i++)
        {
            int k = ram.Next(10);
            result = result + arry[k];
        }
        return result;
    }
}

不能显示！