设置壁纸两次时，GDI +中发生了一般错误

Question

我有这个应该设置壁纸的代码

byte[] data = webClient.DownloadData("http://imageurl");
Image yourImage = null;
using (MemoryStream mem = new MemoryStream(data))
{
    yourImage = null;
    yourImage = Image.FromStream(mem);
    yourImage.Save("image.jpg", ImageFormat.Jpeg);
    Wallpaper.Set(new Uri("image.jpg", UriKind.Relative), Wallpaper.Style.Stretched);
    yourImage.Dispose();
}

每当我运行两次时，它会在yourImage.Save("image.jpg", ImageFormat.Jpeg);处停止，说它是

System.Runtime.InteropServices.ExternalException was unhandled
  HResult=-2147467259
  Message=Allgemeiner Fehler in GDI+.
  Source=System.Drawing
  ErrorCode=-2147467259
  StackTrace:
       bei System.Drawing.Image.Save(String filename, ImageCodecInfo encoder, EncoderParameters encoderParams)
       bei System.Drawing.Image.Save(String filename, ImageFormat format)
       bei StreetViewHintergrundbild.Form1.button1_Click(Object sender, EventArgs e) in d:\Form1.cs:Zeile 70.
       bei System.Windows.Forms.Control.OnClick(EventArgs e)
       bei System.Windows.Forms.Button.OnClick(EventArgs e)
       bei System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
       bei System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
       bei System.Windows.Forms.Control.WndProc(Message& m)
       bei System.Windows.Forms.ButtonBase.WndProc(Message& m)
       bei System.Windows.Forms.Button.WndProc(Message& m)
       bei System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       bei System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       bei System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       bei System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       bei System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
       bei System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       bei System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       bei System.Windows.Forms.Application.Run(Form mainForm)
       bei StreetViewHintergrundbild.Program.Main() in d:\Program.cs:Zeile 19.
       bei System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       bei System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       bei Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       bei System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       bei System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       bei System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

Wallpaper.cs：

using Microsoft.Win32;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace ns
{
    public sealed class Wallpaper
    {
        Wallpaper() { }

        const int SPI_SETDESKWALLPAPER = 20;
        const int SPIF_UPDATEINIFILE = 0x01;
        const int SPIF_SENDWININICHANGE = 0x02;

        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        static extern int SystemParametersInfo(int uAction, int uParam, string lpvParam, int fuWinIni);

        public enum Style : int
        {
            Tiled,
            Centered,
            Stretched
        }

        public static void Set(Uri uri, Style style)
        {
            System.IO.Stream s = new System.Net.WebClient().OpenRead(uri.ToString());

            System.Drawing.Image img = System.Drawing.Image.FromStream(s);
            string tempPath = Path.Combine(Path.GetTempPath(), "wallpaper.bmp");
            img.Save(tempPath, System.Drawing.Imaging.ImageFormat.Bmp);

            RegistryKey key = Registry.CurrentUser.OpenSubKey(@"Control Panel\Desktop", true);
            if (style == Style.Stretched)
            {
                key.SetValue(@"WallpaperStyle", 2.ToString());
                key.SetValue(@"TileWallpaper", 0.ToString());
            }

            if (style == Style.Centered)
            {
                key.SetValue(@"WallpaperStyle", 1.ToString());
                key.SetValue(@"TileWallpaper", 0.ToString());
            }

            if (style == Style.Tiled)
            {
                key.SetValue(@"WallpaperStyle", 1.ToString());
                key.SetValue(@"TileWallpaper", 1.ToString());
            }

            SystemParametersInfo(SPI_SETDESKWALLPAPER,
                0,
                tempPath,
                SPIF_UPDATEINIFILE | SPIF_SENDWININICHANGE);
        }
    }
}