Neil Turner's Blog

Blogging about technology and randomness since 2002

ImageOptim – even better image compression

Screenshot of ImageOptim

It’s been a couple of years since I wrote about ImageOptim – here’s my App of the Week review from May 2011. It’s a Mac frontend for a variety of tools for losslessly compressing image files – PNG, GIF and JPEG are all supported.

In the time since then the app has been updated; most recently with support for zopfli which further improves the compression of PNG images (or indeed any zip file for that matter). In my experience, zopfli is better than the other tools that ImageOptim supports – OptiPNG, PNGOUT, AdvPNG and PNGCrush – around two out of three times. However, ImageOptim will use all of them and then keep whichever resultant file is the smallest.

With me wiping and re-loading all of the data on the server recently, it’s given me the opportunity to run all of my old images through ImageOptim. Whilst this has only resulted in a few more megabytes of disk space being freed up, it will hopefully reduce the amount of data downloaded and make images appear more quickly on slower connections. In some cases, files which I had already put through ImageOptim in the past could be compressed even further, thanks to zopfli.

ImageOptim is free and open-source, and available to download for Mac computers from

From the same developer is another tool called ImageAlpha, which reduces the number of colours used in PNG files, and therefore reduces the file size. It’s separate because, unlike ImageOptim, it offers ‘lossy’ compression – the resultant file will lose some quality in the process. But, unless you zoom right in, it’s barely noticeable, and some significant file size reductions are possible. Plus, it can work in tandem with ImageOptim to make really small images. The screenshot used in this blog post has been through both tools and therefore the full-size image is only 23 kilobytes, rather than 185 as it would be uncompressed. I haven’t used ImageAlpha on many of my old, existing images – just those that were already quite large – but I have been using it for a few months on any new images that I’ve uploaded.

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