[Gimp-user] how / when to use gimp-image-delete

[Gary Aitken <gimp dreamchaser org>]

When processing a series of files in script-fu,
I'm trying to do the following:

for each file name:
  Create an image by reading it in.
    (set! orgImg (car (file-ufraw-load ...
  Create another image by duplicating the input image and manipulating it.
    (set! newImg (car (gimp-image-duplicate orgImg)))
    (gimp-image-scale newImg wid ht)
    (plug-in-unsharp-mask RUN-NONINTERACTIVE newImg newImg ...
  Write the new image out.
    (file-jpeg-save ... newImg ...
  Since I'm done with the newly created image, I did a
    (gimp-image-delete newImg)

Continuing on, 
if I try to re-use that variable to create another image
from the original image
and try to manipulate it, I get an error after a bit:

    (set! newImg (car (gimp-image-duplicate orgImg)))    ; doesn't complain
    (gimp-image-scale newImg wid ht)                     ; doesn't complain
    (plug-in-unsharp-mask RUN-NONINTERACTIVE newImg newImg ...
  Error: Procedure execution of plug-in-unsharp-mask failed on invalid input arguments: Procedure 
'plug-in-unsharp-mask' has been called with an invalid ID for argument 'drawable'. Most likely a plug-in is 
trying to work on a layer that doesn't exist any longer. 

If I comment out the 
  gimp-image-delete
it works fine.

Can someone explain to me what is going on?
(Using gimp 2.6)

Thanks,

Gary