gimp script to batch invert

test-batch-invert.py

#!/usr/bin/env python
#
# -------------------------------------------------------------------------------------
#
# Copyright (c) 2013, Jose F. Maldonado
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import os
from gimpfu import *

def batch_invert(img, layer, inputFolder, outputFolder):
    ''' Inverts the colors of the PNG and JPEG images of a folder.
    
    Parameters:
    img : image The current image (unused).
    layer : layer The layer of the image that is selected (unused).
    inputFolder : string The folder of the images that must be inverted.
    outputFolder : string The folder in which save the inverted images.
    '''
    # Iterate the folder
    for file in os.listdir(inputFolder):
        try:
            # Build the full file paths.
            inputPath = inputFolder + "\\" + file
            outputPath = outputFolder + "\\" + file
        
            # Open the file if is a JPEG or PNG image.
            image = None
            if(file.lower().endswith(('.png'))):
                image = pdb.file_png_load(inputPath, inputPath)
            if(file.lower().endswith(('.jpeg', '.jpg'))):
                image = pdb.file_jpeg_load(inputPath, inputPath)
                
            # Verify if the file is an image.
            if(image != None):
                # Invert the image.
                if(len(image.layers) > 0):
                    layer = image.layers[0]
                    pdb.gimp_invert(layer)
                    
                    # Save the image.
                    if(file.lower().endswith(('.png'))):
                        pdb.file_png_save(image, image.layers[0], outputPath, outputPath, 0, 9, 0, 0, 0, 0, 0)
                    if(file.lower().endswith(('.jpeg', '.jpg'))):
                        pdb.file_jpeg_save(image, layer, outputPath, outputPath, 0.9, 0, 0, 0, "Creating with GIMP", 0, 0, 0, 0)
        except Exception as err:
            gimp.message("Unexpected error: " + str(err))

register(
    "python_fu_test_batch_invert",
    "Batch invert",
    "Inverts the colors of the PNG and JPEG images of a folder",
    "JFM",
    "Open source (BSD 3-clause license)",
    "2013",
    "<Image>/Filters/Test/Batch invert",
    "*",
    [
        (PF_DIRNAME, "inputFolder", "Input directory", ""),
        (PF_DIRNAME, "outputFolder", "Output directory", "")
    ],
    [],
    batch_invert)

main()

Hey man, @HerbFargus
I am having a problem with installing this plugin into gimp. I have never used gimp and plugins for it. I followed the tutorials online to copy the .py file into plugins folder but it does not show up in the python console browse.
Any tip on how to solve it, would be appreciated.

Put the .py file into the plugins folder (on windows for me it's C:\Users\USERNAME.gimp-2.8\plugins)

Then under the filters I have it under Test>batch invert then you should be able to go from there.