asp.net GDI+把图片绘制成自定义的椭圆形状

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Drawing;
using System.Drawing.Drawing2D;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        Bitmap bitmap = new Bitmap(400, 200);
        Graphics g = Graphics.FromImage(bitmap);
        g.Clear(Color.White);
        TextureBrush mytextbrush = new TextureBrush(System.Drawing.Image.FromFile(Server.MapPath("~/images/001aa.jpg")));
        g.FillEllipse(mytextbrush,0,0,400,200);
        System.IO.MemoryStream ms=new System.IO.MemoryStream();
        bitmap.Save(ms,System.Drawing.Imaging.ImageFormat.Jpeg);
        Response.ClearContent();
        Response.ContentType = "image/Jpeg";
        Response.BinaryWrite(ms.ToArray());
    }
}