Google Chrome Labs, which develops experimental web apps, has released an advanced browser-based optimization tool to get your images ready for the web.
Once Squoosh has loaded, you can use it without an internet connection as it performs all its work within the browser itself. To get started with, just drop an image into your browser. Google says it's best used with Chrome (naturally), but it works with all modern web browsers, including mobile ones.
Squish with Squoosh
Squoosh can convert and optimize images for a variety of purposes, with formats including OptiPNG, MozJPG, WebP, and Browser PNG, JPG, and WebP. You can resize the image, and reduce the color palette, with advanced options for dithering, smoothing and sampling (the exact settings available will depend on your chosen output format).
You can preview the results of the optimization in real time, using a slider to see the image before and after. The converted file size will also be updated in real time, and you can download the optimized image using the button on the bottom left once you've finished.
Squoosh is open source, and Google Chrome Labs has made the source code available on GitHub.