silverlight中的圖片控件沒有直接將圖片內容保存到文件的方法，也不能直接讀出圖片內容，需要借助silverlight3.0新增的一個類WriteableBitmap來保存。代碼如下：

public static void SaveAsPNG(Image img)
         {
SaveFileDialog sfd = new SaveFileDialog()
             {
                 DefaultExt =  "png",
                 Filter =  "Png files (*.png)|*.png|All files (*.*)|*.*",
                 FilterIndex = 1
             };
             if (sfd.ShowDialog() == true)
             {
var bitmap = new WriteableBitmap((BitmapImage)img.Source);
EditableImage imageData = new EditableImage(bitmap.PixelWidth, bitmap.PixelHeight);
             try
             {
                 for (int y = 0; y  < bitmap.PixelHeight; ++y)
                 {
                     for (int x = 0; x  < bitmap.PixelWidth; ++x)
                     {
                         int pixel = bitmap.Pixels[bitmap.PixelWidth * y + x];
                         imageData.SetPixel(x, y,
                         (byte)((pixel  >> 16) & 0xFF),
                         (byte)((pixel  >> 8) & 0xFF),
                         (byte)(pixel  & 0xFF), (byte)((pixel >> 24) & 0xFF)
                         );
                     }
                 }
             }
             catch (System.Security.SecurityException)
             {
                 throw new Exception( "Cannot print images from other domains");
             }
             // Save it to disk
             Stream pngStream = imageData.GetStream();
             byte[] binaryData = new Byte[pngStream.Length];
             pngStream.Read(binaryData, 0, (int)pngStream.Length);
             Stream stream = sfd.OpenFile();
             stream.Write(binaryData, 0, binaryData.Length);
             stream.Close();
}