If you’re on Mac OSX, you have a really nifty cli tool at your disposal: sips. It uses OSX core image processing capabilities, and makes them available for shell scripting or batch image manipulation. Rejoice!

This would be a basic example:
sips -Z 200x200 image.jpg
To resize image.jpg to be 200×200 pixels.

For more info, check
man sips

Have fun!