The best Mac image compressor app (and how it works)

You search the App Store for “image compressor” and you get 200 apps that all promise the same thing. Half of them upload your files to a server. A third are abandoned. The rest are some flavour of useful. Here’s what actually matters when picking one — and what a good Mac image compressor does under the hood.

What a real image compressor does

It’s not magic. Underneath, every reputable image compressor is wrapping the same handful of open-source tools:

• pngquant for PNG palette quantization (60-85% reductions).
• MozJPEG or libjpeg-turbo for JPEG re-encoding (30-50% reductions vs default encoders).
• Zopfli / OptiPNG for genuinely lossless PNG re-compression (10-25% extra).
• gifsicle for GIF optimisation.
• libheif for HEIC conversion.

Apps that don’t ship these tools are typically just running sips (which is fine, but limited to ~15% on PNG) or uploading your files to a remote server that runs the same open-source pipeline before sending the result back.

What to look for

A good Mac image compressor:

1. Runs locally. Your files never leave your Mac.
2. Bundles real codecs — pngquant, MozJPEG, libheif — not just sips.
3. Supports batch. Single-file tools are a hard sell when you have 50 photos.
4. Handles multiple formats in one pass.
5. Doesn’t overwrite originals by default. You should be able to compare before/after.
6. Strips EXIF (location data, camera serial, etc.).
7. Doesn’t make files bigger. Already-optimised images should pass through unchanged.

Free options that tick most of these: ImageOptim (compression only, no resize/conversion). Paid options that tick all of them: Melt.

Why Melt

Download Melt is $9.99 one-time. It bundles pngquant, MozJPEG-equivalent encoding, libheif, and the rest of the standard pipeline. Drag-and-drop, batch by default, non-destructive output, EXIF strip, “never makes it bigger” safety check, and format conversion (HEIC to JPEG, PNG to JPEG, etc.) all in one window.

1. Download Melt and open it.
2. Drag images or a folder in.
3. Pick output format and quality.
4. Optionally enable Resize.
5. Click Compress.

The full feature set is what makes the case for paying $10 over running ImageOptim. If you only ever need compression of one format, ImageOptim is fine. If you regularly do mixed jobs — “compress these screenshots, convert these HEICs, resize these for the website, strip EXIF on everything” — Melt is faster.

How much smaller will it really get?

For typical real-world images:

• Photos as JPEG at quality 80: 50-70% smaller.
• UI screenshots as PNG quantized: 70-85% smaller.
• iPhone HEIC converted to compressed JPEG: roughly the same size (HEIC is already efficient), but compatible with everything.

The right tool is the one that does what you need without making you think. Pick the one whose UI matches the way you work.