It is because the Creation was delayed. If you want the picture to be loaded immediately, you can simply add this code into the init phase.
src.CacheOption = BitmapCacheOption.OnLoad;
like this:
src.BeginInit();
src.UriSource = new Uri("picture.jpg", UriKind.Relative);
src.CacheOption = BitmapCacheOption.OnLoad;
src.EndInit();