%@ Page Language="C#" AutoEventWireup="true" CodeBehind="GetPictureByUrl.aspx.cs" Inherits="HoverTreeMobile.GetPictureByUrl" %>
!DOCTYPE html>
html xmlns="http://www.w3.org/1999/xhtml">
head runat="server">
meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
 meta name="viewport" content="width=device-width, initial-scale=1" />
 title>根据网址把图片下载到服务器/title>
/head>
body>
 form id="form1" runat="server">
 div>
 图片网址：br />asp:TextBox runat="server" ID="textBoxImgUrl" Width="500" Text="/hvtimg/201508/cnvkv745.jpg" />
  br /> asp:Button runat="server" ID="btnImg" Text="下载" OnClick="btnImg_Click" />
  br />asp:Image runat="server" ID="hvtImg" />
  br />
  asp:Literal runat="server" ID="ltlTips" />
 /div>
 /form>
/body>
/html>

using System;

namespace HoverTreeMobile
{
 public partial class GetPictureByUrl : System.Web.UI.Page
 {
  protected void Page_Load(object sender, EventArgs e)
  {

  }

  protected void btnImg_Click(object sender, EventArgs e)
  {
   try
   {
    System.Net.WebClient m_hvtWebClient = new System.Net.WebClient();
    string m_keleyiPicture = Server.MapPath("/hovertreeimages/keleyi.jpg");

    //根据网址下载文件
    m_hvtWebClient.DownloadFile(textBoxImgUrl.Text, m_keleyiPicture);

    hvtImg.ImageUrl = "/hovertreeimages/keleyi.jpg";
    ltlTips.Text = string.Empty;
   }
   catch(Exception ex)
   {
    ltlTips.Text = ex.ToString();
   }

  }
 }
}

using System.Net;
      WebClient myclient = new WebClient();
      myclient.DownloadFile("http://www.baidu.com/img/sslm_logo.gif",@"c:\baidu.gif"); 
DownloadFile方法里的address就是你要拼成的远程服务器上的URL.