Short answer: a mass image compressor for Mac shrinks the file size of hundreds of images in one go, on your own computer. macOS can do small batches for free with Preview and the sips command. For whole folders, saved presets and more control, a native app does it locally, so your photos never leave your Mac.
I do this most weeks. Images for my blog, so the pages load fast. And design assets: the Picmal press kit has files that are too big for the upload limits of many directories. I drop the folder in, compress all of it, and upload.
Why compress in bulk
The piles look like this. A holiday folder too big to email. Product photos a website keeps rejecting. An iCloud plan that’s full. A shoot a client wants as a download link instead of a drive.
Same fix every time: make the files smaller, all at once, without wrecking them.
Two levers do it. Compression keeps the image the same size on screen and stores it more efficiently. Resizing makes the image smaller in pixels. Most piles need a bit of both. If you want to bulk resize images on Mac too, I covered that in its own guide.
Local Mac app vs online compressors
Online compressors do one thing well: they need nothing installed, and for five files they’re quick.
Past that, the costs add up. You upload every photo to someone’s server, wait, and download a zip. Free tiers cap the file size and the number of files. And they stop working the moment your connection does.
A mass image compressor for Mac that runs locally skips all of it. The work happens on your own processor. It’s limited only by your disk, it works offline, and it fits into Finder, so you never open a browser. For more on the privacy side, see converting images without uploading them online.
How to mass compress images on a Mac
The free way: Preview
- Select the images in Finder and open them all in Preview.
- Select every thumbnail in the sidebar.
- Choose File > Export Selected Images, then click Options.
- Pick JPEG and drag the Quality slider down.
It works for a few dozen photos. Beyond that it slows down, and you get one setting for everything.
The free way: Terminal
mkdir smallersips -s format jpeg -s formatOptions normal *.jpg --out ./smallerformatOptions takes low, normal, high or best. It’s fast and has no limit, but there’s no preview, and a typo in the path costs you ten minutes.
With a native Mac app
- Choose an app that compresses locally, in batches. Check that it keeps your folder structure and your photos’ metadata.
- Drag the whole folder in. Subfolders included.
- Pick a preset. One for web, one for email, one for archiving. Set it once, reuse it.
- Run it, then open a few results at full size before you delete anything.
In Picmal that’s the whole flow. It batch processes hundreds of files at once, keeps your settings as presets, and can watch a folder so anything you drop in gets compressed with your preset. You can also compress a lot of pictures at once straight from Finder, with the right-click Quick Action.
I almost always use the Balanced preset. For blog images and press kit assets, I rarely need anything else.
Common mistakes when compressing images in bulk
- Over-compressing. Fine detail, smooth gradients like a sky, and text in screenshots break first. Pick a level, then check those spots.
- Not testing a sample. Run five images before you run five hundred. Five minutes now saves a redo later.
- Compressing your only copy. Write the results to a new folder. Keep the originals until you’ve looked at the output.
- Confusing resizing with compressing. A photo straight off a camera is far bigger in pixels than any screen will show it. Resizing it to the size it’ll be displayed at often saves more than any compression setting.
Frequently asked questions
What is the best image compressor app for Mac?
It depends on volume. For a few photos, Preview is free and already installed. For web work, ImageOptim is a well-loved free option that shrinks files without changing how they look. For hundreds of mixed files with presets, folder watching and Finder integration, a native batch app like Picmal fits better.
How do I reduce the MB size of a photo on a Mac?
Open it in Preview, choose File > Export, pick JPEG, and lower the Quality slider. The dialog shows the new file size as you drag. Resizing helps even more: Tools > Adjust Size. A photo shown on a website rarely needs the full resolution your camera saved. More in how to reduce image file size on Mac.
How to compress a lot of pictures at once?
Select them all in Preview and use File > Export Selected Images with a lower JPEG quality. Or run the sips command on the folder in Terminal. For hundreds of pictures, a batch app is quicker to set up: drag the folder in, pick a preset, and let it run while you do something else.
How to mass resize images on Mac?
In Finder, Quick Actions > Convert Image lets you pick small, medium or large for a whole selection. In Preview, Tools > Adjust Size works on every selected image at once. For exact sizes across a folder, use sips in Terminal or a batch app. I wrote a full guide to bulk resizing images on Mac.
Is there a free mass image compressor for Mac?
Yes. Preview and the sips command are free and built in, and ImageOptim is a free download. Online compressors are free too, with limits and an upload. Paid local apps like Picmal add batches of hundreds, presets and folder watching. Picmal has a 7-day free trial, so you can test it on your own files first.
A native app for it
If you want a native Mac app that does this locally, I built Picmal for exactly this. It converts and compresses images, video, audio and PDFs on your Mac, and it’s bought once, with a 7-day free trial. John Voorhees reviewed it for MacStories in November 2025, if you’d like a second opinion.
