Batch conversion from JPG to PNG

[pygmalion]

Well, the whole original process is actually
scanned file - convert to grayscale - curve correction - reduce size - convert to PNG

By testing I determined that convert to PNG part makes huge difference - manual procedure gives much smaller document than batch procedure.

So I tried
scanned file - convert to grayscale - curve correction - reduce size - convert to PNG - remove alpha
which gives better result, but not as good as manual procedure.

However
scanned file - convert to grayscale - curve correction - reduce size - convert to indexed - convert to PNG - (remove alpha)
gives a better result, depending on the number of colors.  Interestingly, in this procedure, remove alpha makes no difference.