I doubt renaming files from *.png to *.jpg actually does any conversion (at least via mv) — png and jpg are two very different image formats. A handy-dandy command-line utility for manipulating images is imagemagick. You can use apt-get on linux or brew install on osx to install it on your system. Afterwards, you can batch convert like so:
for i in *.png ; do convert "$i" "${i%.*}.jpg" ; done
where convert is part of the imagemagick toolbox. You will still want to verify by hand a couple of images that the conversion went thru as expected (sometimes, pngs with transparent background can confuse imagemagick — google if you are stuck).