Save BitmapImage to File

I am working on a program that downloads images from a URL to a bitmapimageand displays it. Next I try to save the bitmapimage to the harddrive using jpegbitmapencoder. The file is successfully created but the actual jpeg image is empty or 1 black pixel.

public Guid SavePhoto(string istrImagePath)
{
    ImagePath = istrImagePath;

    BitmapImage objImage = new BitmapImage(
        new Uri(istrImagePath, UriKind.RelativeOrAbsolute));
    PictureDisplayed.Source = objImage;
    savedCreationObject = objImage;

    Guid photoID = System.Guid.NewGuid();
    string photolocation = photoID.ToString() + ".jpg";  //file name

    JpegBitmapEncoder encoder = new JpegBitmapEncoder();
    encoder.Frames.Add(BitmapFrame.Create(objImage));

    using (FileStream filestream = new FileStream(photolocation, FileMode.Create))
    {
        encoder.Save(filestream);
    }

    return photoID;
}

This is the function that saves and displays the photo. The photo is displayed correctly but again when it is saved I get an empty jpeg or 1 black pixel.