How to resize images using terminal on Mac OSX?

I need a simple and free way to resize images and do batch jobs, if necessary. Free image manipulation software has been trickier to use than it should be.

As pointed out by LifeHacker, the following command will do this very easily:

sips -Z 640 *.jpg

To quote their explanation:

"sips is the command being used and -Z tells it to maintain the image's aspect ratio. "640" is the maximum height and width to be used and "*.jpg" instructs your computer to downsize every image ending in .jpg. It's really simple and shrinks your images very quickly. Be sure to make a copy first if you want to preserve their larger size as well."

Source: http://lifehacker.com/5962420/batch-resize-images-quickly-in-the-os-x-terminal

imagemagick helps:

$ convert foo.jpg -resize 50% bar.jpg

There are a lot more things it can do, including the conversion between formats, applying effects, crop, colorize and much, much more.