Imagemagick is an amazing open-source application that lets you perform all sorts of image & video processing from the command line. On OSX, to install imagemagick, use brew

$ brew install imagemagick

I often use imagemagick to resize images for blog posts, emails, etc. There are two different ways I use it to resize an image:

Resize Images By Percentage

To resize an image by percentage, simply execute the following command

$ convert IMAGE.PNG -resize 50% NEW_IMAGE.PNG

Resize Images By Dimensions

To resize an image by image dimension, simply execute the following command

$ convert IMAGE.PNG -resize 64x64 NEW_IMAGE.PNG