Your answers are:
jpegoptim
http://encode.ru/threads/1587-jpegop...ll=1#post33541 for jpegs.
Pngout + advpng with zopfli algorithm for png.
And advzip for the cbz.
that's for lossless recompression and cleaning.
That's the 1st I'd try. If it's not enough then one can always start small steps on the lossy road.