convert rose.jpg rose.png
convert rose.jpg -resize 50% rose.png

convert rose.jpg -density 180 rose.jpg
$ identify -format "%x x %y" rose.jpg
180 PixelsPerInch x 180 PixelsPerInch

find . -name "*.jpg"   | awk '{print "convert " $0 " -density 72 " $0}' | sh

find . -name "*.jpg"   | awk '{print "identify -format \" %x x %y\" " $0}' | sh

find -name "*.jpg" -exec convert "s/.jpg/.png/g" \{\} \;