Having to open a whole bunch of images just to resize and save them again is a pain. Luckily, Apple’s built-in Automator tool can streamline this process, letting you select a group of images and resize them all at once—automatically.
Step One: Set Up a New Service
First, launch Automator (Command+Space, then type “Automator”), and create a new service.
Services are like little programs that you can run by right-clicking a file, or from the app’s menu.
The first thing you need to set up a service is an input, and our input will be image files.
Now that you’ve set the input, you’ll tell Automator what to do with files.
In the search box at the top of the left pane, type “Scale Images” and then drag the “Scale Images” action into the main panel. Automator will prompt you to add a block to copy files over before resizing, which is handy to prevent accidental resizing, such as missing a 0 and losing all the image quality. By default, the action will just make copies of the resized images and save them to the desktop (or another folder of your choice). If you want to resize them in place and replace the originals, leave this copy block out.
You should now have two actions in the service. To make it work, click “Options” on the Scale Images service, and select “Show this action when the workflow runs.”
This will make the service open a dialog asking for the image size. If you’d rather stick to a single size, you can leave this unchecked and type in the size you always want to use.
When you’re done, save the service and name it whatever you’d like—we’re going with “Resize.”
Step Two: Put Your New Service to Use
Right click on any image, point to the “Services” option at the bottom of the context menu, and then choose “Resize”—or whatever you named your service.
You should see it copy the file over and then resize it. Bear in mind that the scale function resizes based on aspect ratio. If your image is landscape-oriented, it’ll resize the width; if it’s portrait-oriented, it’ll resize the height.
You can also assign a keyboard shortcut to the service if you want by heading to System Preferences > Keyboard > Shortcuts > Services.