tested on: Linux laptop 3.16.0-4-686-pae #1 SMP Debian 3.16.7-ckt20-1+deb8u2 (2016-01-02) i686 GNU/Linux

if using one core is enough:

apt-get update;

# install image manipulation programs -- binaries
apt-get install imagemagick;
for i in *.png ; do convert "$i" "${i%.*}.jpg" ; done
# or
ls -1 *.png | xargs -n 1 bash -c 'convert "$0" "${0%.*}.jpg"'

if you have multiple cores and want to use them:

apt-get install parallel;
ls -1 *.png | parallel convert '{}' '{.}.jpg'