jpegoptim can reduce size of JPG files. if used without -m option it’s lossless.
apt-get install jpegoptim
find . -type f -exec jpegoptim -m80 {} ‘;’
for pngs there are few tools out of which i found pngcrush to give best results. unfortunately it does not work ‘in-place’ and temporary file needs to be created for each processed image.