optipng

optimize png file

install

centos7

$ sudo yum install optipng

how to use

$ optipng -o7 sample.png

-o7 level7(max)

with backup

$ optipng -o7 -keep sample.png

then create sample.png.bak too

bulk compress

$ find . -iname *.png -type f | xargs optipng -o7