I would be much less reluctant if we make this a new kwarg option.  e.g. `pad_image` rather than `noise_pad`.  

I mostly don't like the meaning of `noise_pad` changing so dramatically depending on whether the image is given as a GalSim Image object or a file from which the image is read in.  It seems to me that those should just be different ways of providing the image.

If we add a new kwarg for the different functionality, then I would want that to also accept either an Image instance or a file name and then do a consistent thing with the resulting image.