public BitmapImage ConvertWriteableBitmapToBitmapImage(WriteableBitmap wbm)
{
    BitmapImage bmImage = new BitmapImage();
    using (MemoryStream stream = new MemoryStream())
    {
        PngBitmapEncoder encoder = new PngBitmapEncoder();
        encoder.Frames.Add(BitmapFrame.Create(wbm));
        encoder.Save(stream);
        bmImage.BeginInit();
        bmImage.CacheOption = BitmapCacheOption.OnLoad;
        bmImage.StreamSource = stream;
        bmImage.EndInit();
        bmImage.Freeze();
    }
    return bmImage;
}

public static class ImageHelpers
{
    public static BitmapImage ToBitmapImage(this WriteableBitmap wbm)
    {
        BitmapImage bmImage = new BitmapImage();
        using (MemoryStream stream = new MemoryStream())
        {
            PngBitmapEncoder encoder = new PngBitmapEncoder();
            encoder.Frames.Add(BitmapFrame.Create(wbm));
            encoder.Save(stream);
            bmImage.BeginInit();
            bmImage.CacheOption = BitmapCacheOption.OnLoad;
            bmImage.StreamSource = stream;
            bmImage.EndInit();
            bmImage.Freeze();
        }
        return bmImage;
    }
}