string url = "http://avatar.csdn.net/A/2/6/2_yefengzhixia.jpg";
            string filepath = "D:\\pic.jpg";
            WebClient mywebclient = new WebClient();
            mywebclient.DownloadFile(url, filepath);
            MessageBox.Show("OK");    

<Grid>
        <DockPanel>

            <GroupBox x:Name="groupBox" Header="图片网址" DockPanel.Dock="Top">
                <StackPanel>
                    
                <StackPanel Orientation="Horizontal" Margin="10" >
                    <Button x:Name="button" Content="查看网页中的图片" Margin="5" Click="button_Click"  />
                        <TextBlock Text="网址："></TextBlock>
                    <TextBox x:Name="textBox" Text="http://www.tooopen.com/img/87.aspx" HorizontalAlignment="Left" MaxWidth="600" TextWrapping="Wrap" MinWidth="300"  VerticalAlignment="Top"  />
                </StackPanel>
                    <TextBlock Text="下载信息："></TextBlock>
                    <TextBlock Margin="10" Name="txtcount" Text="{Binding downCount}">
                    </TextBlock>
                    <TextBlock Name="total"></TextBlock>
                    <Button Click="Button_Click_1" Content="打开下载图片文件夹"></Button>
                </StackPanel>
            </GroupBox>
                <ListBox Name="lb"></ListBox>
        </DockPanel>

    </Grid>

 public partial class MainWindow : Window,INotifyPropertyChanged
    {
        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = this;
        }

        private int c;

        public int downCount
        {
            get { return c; }
            set { c = value;
                if (PropertyChanged != null)
                    PropertyChanged(this, new PropertyChangedEventArgs("downCount"));
            }
        }

        public ObservableCollection<string> list = new ObservableCollection<string>();
         

        public event PropertyChangedEventHandler PropertyChanged;

        private async void button_Click(object sender, RoutedEventArgs e)
        {
           
            var path = textBox.Text;
            if (string.IsNullOrWhiteSpace(path))
            {
                textBox.Focus();
                MessageBox.Show("请输入网站地址");
                return;
            }
            HttpClient client = new HttpClient();
            var str = await client.GetStringAsync(new Uri(path));

            Regex rg = new Regex("<img.+?src=http://www.mamicode.com/"(.+?)\"");
            var mth = rg.Matches(str);

            WebClient wc = null;
            int i = 0;
            total.Text="共有图片："+mth.Count;

            Action<string> listboxAddImgName = x => {
                lb.Items.Add(x);
              
            };
            foreach (Match item in mth)
            {
                i++;

                var img = item.Groups[1].Value;
                string Ex = img.Substring(img.LastIndexOf("."));
                string imgName = i + Ex;
                Task.Run(() =>
                {
                    wc = new WebClient();
                    wc.DownloadFile(img, "DownLoadImages/" + imgName);
                    return imgName;
                }).ContinueWith(r=> {
                    downCount +=1;
                    Dispatcher.Invoke(listboxAddImgName,r.Result);
                });

            }

        }

        private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            System.Diagnostics.Process.Start(Environment.CurrentDirectory+"/DownLoadImages");
        }
    }