I'd second ImageOptim on Mac or PNGGauntlet on Windows https://pnggauntlet.com . I'd be careful using pngquant as well, as it will only ever give you 256 or less colour image or your original image back. PNGGauntlet and ImageOptim will optimise your PNGs losslessly.

I tend to use SVGO’s Missing GUI: https://jakearchibald.github.io/svgomg/. (Great demonstration of service workers for offline support, too.)

For Linux users I made https://trimage.org

Thanks, I'll try it.

On Windows, I only need to optimize images occasionally and tend to use the free version of https://kraken.io There are probably better solutions, but this is very easy and no install needed.

On Mac, you're definitely right that ImageOptim is the best.

What would you recommend for an iOS app written in Swift that has to optimize images drag-and-dropped on it by the user?

Because it suits their workflow?

Because it adheres to the company's IT policies?

Because they like to review results instead of relying on some black box automation?

Because they like to work in-house and not on someone else's cloud?

Just off the top of my head. I'm sure there are a thousand other reasons.

svgcleaner is supposed to be faster and have a better compression ratio than svgo: https://github.com/RazrFalcon/svgcleaner

It also has a GUI.

svgcleaner looks not to support any lossy compression at this stage. Granted, svgo’s lossy compression is very pathetic, but for many images it will have enormous impact. (I say very pathetic, because it’s pretty much just rounding of numbers to N decimal places—no awareness of perception or how different scales may mean that one decimal place means radically different things in different parts of the image, no path simplifications using Visvalingam-Whyatt or similar, &c.)

Being focused on theoretically lossless compression techniques, svgcleaner’s GUI doesn’t look to include rendering of the SVG, and focuses on batch operation. For svgo there is Jake Archibald’s SVGOMG which renders the SVG and so lets you see what the impact of the lossy compression employed.

After fighting with the options, failing to find a good default, and still consistently getting larger images, I fail to see how svgcleaner is better.

I think it's only with lossless compression that it's better.

svgo has a gui online called SVGOMG.