[WebMethod]
public string GetPicture() {
  // Load the picture
  Image img = Image.FromFile("Path/To/Picture.jpg");
  // Create a memory stream, and put the image contents into it.
  MemoryStream stream = new MemoryStream();
  img.Save(stream, img.RawFormat);
  // return data, encoded in Base64
  return Convert.ToBase64String(stream.ToArray());
}