I started my mod with the same thoughts and ended up editing the images one by one. Part of the reason are the pinky lines which have to stay the way they are exactly regarding colour tone and brightness. Most batch processors have the basic capabilities to do job except ignoring those pink lines. For basic image management and some editing / converting I can highly recommend FastStone ImageViewer.
One advantage of the by hand solution is the raising lust for more. Meaning you get the hang for it step by step and soon you start writing Walter code.