注意：并没有实现CSS中的图片采集，且图片的正则还有待完善。
代码如下：

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;

//引入空间
using System.Net;
using System.IO;
using System.Text;
using System.Text.RegularExpressions;
using System.Collections;

/// <summary>
/// 采集
/// </summary>
public class caiji
{
public caiji()
{
//
// TODO: 在此处添加构造函数逻辑
//
}

/// <summary>
/// 要采集的网页的连接地址
/// </summary>
/// <param name="url">url</param>
/// <returns></returns>
public static string caijiByUrl(string url,string chargest,string path)
{
string str = GetSourceTextByUrl(url,chargest);

ArrayList lib = new ArrayList();

int i = 0;
//根据url取得网站域名
Uri uri = new Uri(url);

//Scheme或者协议，一般为http,Host为取得域名
string baseurl = uri.Scheme + "://" + uri.Host + "/";

//提取出url，包括src等信息
///S匹配任何非空白字符
Regex g = new Regex(@"(src=(""|/')/S+/.(gif|jpg|png|bmp)(""|/'))", RegexOptions.Multiline | RegexOptions.IgnoreCase);

MatchCollection m = g.Matches(str);

foreach (Match math in m)
{
//已经提取到图片的路径了，但还需要分绝对路径，相对路径，以及后缀名是否为图片，因为可能为.asp,.aspx这些，比如验证码图片
string imgUrl = math.Groups[0].Value.ToLower();//转成小写,=号之间可能有不定的空格

//去除src与单引号，双引号
imgUrl = imgUrl.Replace("src","");
imgUrl = imgUrl.Replace("/"","");
imgUrl = imgUrl.Replace("'","");
imgUrl = imgUrl.Replace("=","");
imgUrl = imgUrl.Trim();

//路径处理
if (imgUrl.Substring(0, 4) != "http")
{
//需要判断是否是绝对路径还是相对路径
if (imgUrl.Substring(0, 1) == "/")
{
imgUrl = baseurl + imgUrl;
}
else
{
imgUrl = url.Substring(0,url.LastIndexOf("/") + 1) + imgUrl;
}
}

//判断元素是否已经存在,-1为不存在
if (lib.IndexOf(imgUrl) == -1)
{
lib.Add(imgUrl);
}
}

string str_ = string.Empty;
WebClient client = new WebClient();

for (int j = 0; j < lib.Count; j++)
{
string savepath = path + DateTime.Now.Month + DateTime.Now.Day + DateTime.Now.Minute + DateTime.Now.Second + j + lib[j].ToString().Substring((lib[j].ToString().Length) -4,4);
try
{
client.DownloadFile(new Uri(lib[j].ToString()), savepath);
str_ += lib[j].ToString() + "<br /> 保存路径为：" + savepath + "<br /><br />";
}
catch (Exception e)
{
str_ += e.Message;
}

}

return str_;
}

public static string GetSourceTextByUrl(string url,string chargest)
{
WebRequest request = WebRequest.Create(url);
request.Timeout = 20000;//20秒超时
WebResponse response = request.GetResponse();

Stream resStream = response.GetResponseStream();
StreamReader sr = new StreamReader(resStream,Encoding.GetEncoding(chargest));
return sr.ReadToEnd();
}
}

使用：比如我是保存到upload文件夹中的：
代码如下：

string path = Server.MapPath("~/upload/");
Response.Write(caiji.caijiByUrl(http://www.jb51.net, "utf-8", path));