How to convert a image to text Windows Phone 7 and else?

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C#

I'm trying convert but it dont handle: WriteableBitmap bmp = new WriteableBitmap((BitmapSource)myImage.Source)

public string ImageToBase64(Image myImage)
        {
            byte[] data;
            WriteableBitmap bmp = new WriteableBitmap((BitmapSource)myImage.Source);

            using (MemoryStream stream = new MemoryStream())
            {
                bmp.SaveJpeg(stream, bmp.PixelWidth, bmp.PixelHeight, 0, 100);
                data = new byte[stream.Length];
                // Read the entire file and then close it
                stream.Read(data, 0, data.Length);
                stream.Close();
            }
            string base64 = System.Convert.ToBase64String(data);
            return base64;
        }