how to cache third party images in silverlight .

hi,

We are showing Video with slides(images) in silverlight .Our images come from amazon server now we want to cache these images before    starting  Video.

our code is given below:

  private void LoadImage(string ImageURL)
        {

            WebClient downloader = new WebClient();
            downloader.OpenReadCompleted += new OpenReadCompletedEventHandler(downloader_OpenReadCompleted);
            string fileName = ImageURL;
            downloader.OpenReadAsync(new Uri(fileName, UriKind.Absolute));
        }


        void downloader_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
        {
             
           //  Create a new BitmapImage and load the stream 

   if(e.Error ==null)

       {    

           BitmapImage loadedImage = new BitmapImage();
            loadedImage.SetSource(e.Result);

             
        }

But it gives following Error in e.Error

System.Reflection.TargetInvocationException was unhandled by user code
  Message=An exception occurred during the operation, making the result invalid.  Check InnerException for exception details.
  StackTrace:
       at System.ComponentModel.AsyncCompletedEventArgs.RaiseExceptionIfNecessary()
       at System.Net.OpenReadCompletedEventArgs.get_Result()
       at Video_PowerPoint.Session.downloader_OpenReadCompleted(Object sender, OpenReadCompletedEventArgs e)
       at System.Net.WebClient.OnOpenReadCompleted(OpenReadCompletedEventArgs e)
       at System.Net.WebClient.OpenReadOperationCompleted(Object arg)
  InnerException: System.Security.SecurityException
       Message=""
       StackTrace:
            at System.Net.Browser.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state)
            at System.Net.Browser.BrowserHttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
            at System.Net.WebClient.GetWebResponse(WebRequest request, IAsyncResult result)
            at System.Net.WebClient.OpenReadAsyncCallback(IAsyncResult result)
       InnerException: System.Security.SecurityException
            Message=Security error.
            StackTrace:
                 at System.Net.Browser.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult)
                 at System.Net.Browser.BrowserHttpWebRequest.<>c__DisplayClass5.b__4(Object sendState)
                 at System.Net.Browser.AsyncHelper.<>c__DisplayClass2.b__0(Object sendState)
            InnerException: