If you need to quickly resize a bunch of images on a Mac, you don't need to open Photoshop. There's a simple way to do it from the command line, by using the sips command. This is the basic syntax:
Resize a single image
By way of example, to resize a single image to a maximum of 800px (either width or height), you'd use the following. It's important to note that this will replace the existing image, so make a copy if you want to keep the original.
$ sips -Z 800 image.jpg
Resize multiple images
Where sips really comes into it's own is for folders of images. To resize multiple images just use a wildcard ` with the file extension (.jpgin this case). The example below will blast through all the images infolderName`, resizing them to 1200px in a flash.
$ sips -Z 1200 folderName/*.jpg
Resize multiple images with format conversion
Converting the format of the image along with resizing is best done through imagemagick's mogrify command. First, ensure you have imagemagick installed:
$ brew install imagemagick
Then run mogrify . The following command will resize all images from inputFolder to 800px wide jpgs in outputFolder.
$ mogrify -resize 800 -path 'outputFolder/' "inputFolder/*.jpg"
The full list of mogrify options is available here.