Batch Compressing PNGs on a Mac With Automator

You have a folder of PNGs. Each one needs to be smaller. Opening them one by one in Preview and adjusting the size is slow, tedious, and you will not do it for fifty files. What you need is a way to batch compress png mac wide, processing every file in a single action without touching each one. That is what this guide builds: a macOS Quick Action that resizes every PNG in a folder, leaves the originals untouched, and finishes in seconds.

First, understand what this workflow does and what it does not do. The Automator Scale Images action resizes the pixel dimensions of an image. It does not reduce the colour palette. PNG compression is always lossless and always on, but file size scales with pixel count. Doubling the width quadruples the data. Resizing before any other step is the single most effective thing you can do for a PNG on a Mac because macOS has no built-in palette reduction. The Scale Images action gives you the real saving available in the operating system itself.

Create a PNG Resize Quick Action

Open Automator from your Applications folder. Create a new document and choose Quick Action as the type. At the top of the workflow pane, set "Workflow receives current" to "image files" and "in" to "Finder". This tells the Quick Action to appear only when you right click image files in the Finder.

From the library on the left, drag "Scale Images" into the workflow. A dialog asks if you want to add a Copy Finder Items step. Say yes. This is the step that saves your originals.

Now configure Scale Images. Set it to resize by percentage or to a specific pixel dimension. For web use, roughly 1600 pixels on the long edge covers a blog image and 2560 covers full screen. The research is precise: resizing before compressing is never wasted work. The compressor runs on fewer pixels and runs better on the smaller result.

Below Scale Images, check that Copy Finder Items appears above it, not after it. The Copy Finder Items step copies the selected files to a folder you choose, then the Scale Images action modifies the copies. The originals in the source folder never change. If you place Copy Finder Items after Scale Images, the originals are resized and the copies stay large, which is the opposite of what you want.

Save the Quick Action with a name like "Resize PNGs". It now appears under the Services or Quick Actions menu when you right click a PNG.

Automator Resize Images: What the Action Can and Cannot Do

The Scale Images action resizes images. That is all it does. It does not change the file format, the colour type, the bit depth, or the pixel data beyond scaling. The output is still a PNG with the same 24 or 32 bit colour depth. This is a lossless transformation at the resolution level: every pixel in the smaller image is a correctly computed sample of the larger image, but the decoded pixels are not identical to the originals because dimensions changed. That is fine. The goal is a smaller file for display, not archival fidelity.

Do not confuse this with palette reduction. Palette reduction, what pngquant and TinyPNG do, rebuilds the image from an adaptive palette of typically 256 or fewer colours. That collapses thousands of near identical shades into one entry, creating the repetition that DEFLATE compression feeds on. A screenshot can shrink 70 to 85 percent that way. Scale Images cannot do that. If you need palette reduction, you need a different tool.

What Scale Images can do is take a 3000 pixel wide PNG and bring it down to 1600 pixels. The file size drops by roughly the ratio of the pixel counts squared. A 3 megabyte file becomes roughly 850 kilobytes. That is a 72 percent reduction from a single setting.

Protect Original PNG Files First

Copy Finder Items must be the first step after receiving the files. It duplicates the selected images into a destination folder of your choice. The Scale Images action then acts on the duplicates. The originals stay in their original location at full size. If you skip this step, the action modifies the files in place. There is no undo.

Set a Destination for Resized PNGs

Create a folder called "Resized" on your Desktop or in your Documents. When you configure Copy Finder Items, select that folder as the destination. Every run of the Quick Action deposits resized copies there. You can then inspect, compress further, or upload without touching the originals.

Run the Quick Action on PNGs

Select the PNG files in the Finder, right click, and choose Quick Actions then your saved action name. The action runs silently. A progress indicator appears briefly. The resized files appear in the destination folder in seconds.

Mac Quick Action Resize PNG: What to Expect from File Size

Run the Quick Action on a folder of screenshots. The typical result: a 1920 by 1080 pixel screenshot at around 1.5 megabytes becomes a 1600 pixel version around 800 kilobytes if the image content is detailed. Flat UI elements compress better than photographs even after resizing because PNG prediction works best on flat colour and sharp edges. Photographic sensor noise makes the prediction wrong nearly every time, giving small random differences that are close to incompressible. A photograph stored as PNG will still be roughly ten times the size of the equivalent JPEG after resizing.

If the files are already screen resolution, the size difference is smaller. An image displayed at 800 pixels wide and stored at 800 pixels carries exactly the data needed. The research states that for high density screens, serve about twice the display width, roughly 2560 pixels on the long edge for full screen. A blog image needs about 1600 pixels. Stick to those targets and the Quick Action delivers files that are sized for the display.

One caveat: if the image has an alpha channel, truecolour with alpha is PNG 32. Resizing preserves the alpha channel. Paletted PNGs with transparency, which carry transparency in a tRNS chunk, also resize correctly. The action handles both.

When PNG File Size Grows

If your source images are already screen resolution like 800 pixels wide and you scale them to 800 pixels, the file may grow slightly due to re encoding. The correct tool returns the original in that case. The Quick Action does not compare sizes; it always re saves the image. If you run it on already small files, keep the originals.

Compressing a Folder of Images Mac Wide

Select a folder in the Finder and run the Quick Action. The action processes every image file inside the selection. This is how to process a whole folder of PNGs on a Mac without opening each one. Subfolders are not recursed; the action handles only the files directly selected. If you need to recurse, use a Folder Action instead of a Quick Action.

Lossless Compression After Resizing PNGs

The Quick Action resizes. That is step one. Step two is lossless optimisation. Run the resized files through ImageOptim, which bundles several lossless optimisers including optipng and zopflipng. These search harder for the best filter per row and run DEFLATE at maximum effort. Typical savings are 5 to 15 percent on top of the resizing savings, sometimes zero on an already optimised file.

Step three, if you need more, is palette reduction. This is a lossy operation. Keep the original. Go to a tool like TinyPNG or pngquant. Reduce to 256 colours or fewer. The typical saving on a UI screenshot is 70 to 85 percent on top of resizing. The visible failure mode is banding in smooth gradients. If the image has a soft alpha band, use a gentler setting.

The order matters. Resize first, then optimise losslessly, then reduce the palette in steps. Compressing pixels you are about to discard is wasted work. The compressor performs better on the smaller image. Palette reduction cannot be undone, which is why the Copy Finder Items step in your Quick Action is non negotiable.

Do not strip metadata on the originals. Metadata chunks can be removed losslessly from the copies after resizing. GIMP Export As has checkboxes to omit background colour, gamma, layer offsets, resolution, creation time and comments. ImageOptim strips metadata by default.

What this workflow cannot do is match the savings of JPEG for photographs. The honest answer: a photograph stored as PNG is roughly ten times the size of the same photo as JPEG at a quality difference invisible at normal viewing distance. If your folder contains photos, change the format. The Quick Action does not convert formats. Use Preview Export or a batch ImageMagick command for that.

File Size Targets After Resizing a Typical 1920x1080 PNG

PNG Palette Limits and JPEG Alternatives

macOS has no built in palette reduction. If you need to compress a folder of images mac wide and the files are photographs, the honest answer is to change the format to JPEG. The Quick Action does not do that. Use Preview Export with the JPEG Quality slider for a single file, or use sips on the command line for a batch.

If your images include text with soft shadows, palette reduction creates a stair step fringe on the semi transparent edges. The soft alpha band around a rounded logo needs enough palette entries or it snaps into discrete steps. A gentler setting with 256 colours and dithering fixes this on most images, but you lose the extreme savings of a 64 colour palette.

Do not run the Quick Action on originals you plan to print. Print resolution is far higher than screen and exposes banding invisible on a monitor. Send print the untouched original.

This Quick Action exists to solve one problem: how to process a whole folder of PNGs on a Mac without opening each one. It resizes the pixel dimensions. It does not reduce the palette. It is a practical build that every Mac user with a folder of oversized screenshots can set up in five minutes. The Copy Finder Items step is the difference between a safe workflow and a destructive one. Keep it first. Resize the copies. Inspect the results. Compress further if needed. Done.