Using the ImageMagick convert command to convert between image files:
Source: http://www.imagemagick.org/script/convert.php

Convert a pdf to png:

set FILES_IN="FileIn.pdf"
set FILES_OUT="FileOut.png"
convert -density 600x600 -resize 2000x2000 -quality 90 %FILES_IN% %FILES_OUT%

And back when the png file is edited:

set FILES_IN="FileOut.png"
set FILES_OUT="FileOut.pdf"
convert -density 600x600 -resize 2000x2000 -quality 90 %FILES_IN% %FILES_OUT%

Bonus: It can even fill or crop images

convert *.jpg -resize 383x353^^ -gravity center -extent 383x353 *.jpg