ImageOptim

Strips metadata and recompresses PNG and JPEG files in place, without visible quality loss.

What it does

ImageOptim runs a pipeline of optimisers — Zopfli, PNGOUT, OxiPNG, AdvPNG, MozJPEG, Guetzli — and keeps whichever produces the smallest file. It also strips EXIF, colour profiles and thumbnails, which on a photo straight from a phone is frequently the majority of the payload.

Files are replaced in place, so the workflow is dragging a folder onto the window.

Notes

Lossy mode exists and is off by default; leave it off when the source is the only copy.
Stripping EXIF removes GPS coordinates — worth doing on anything about to be published from a home address.
For build-time optimisation Hugo’s own image pipeline (resources.Get plus .Resize/.Process) covers most of this without a manual step.

Alternative to

Alternative	Type	Trade-off
Squoosh	Free	Google’s web-based compressor, with a live quality comparison
TinyPNG	Freemium	The well-known web and API service
oxipng, pngquant and jpegoptim	Open source	The same engines from the command line
Hugo, Sharp or vips in a build step	Open source	Better than optimising by hand at all