http://www.imagemagick.org/script/convert.php

http://www.howtogeek.com/109369/how-to-quickly-resize-convert-modify-images-from-the-linux-terminal/

http://www.imagemagick.org/script/mogrify.php

http://blog.blupixelit.eu/resize-all-images-in-a-folder-using-bash-convert-command/

apt-get install imagemagick