ImageOptim is a really cool tool for optimising image data, the latest version also includes the Zöpfli compression algorithm, developed by some colleagues. Basically you drag/drop the
PNG files you want to use on your web-site and the tool will reduce them as much as possible, the results are pretty impressive.
For instance, I’m using the Sixteen theme for WordPress, which is nice, but the images have not been optimised. By running the tool, I brought down the size of the image set by 74K (out of 465). You can download the optimised images here. They are pixel-identical to the ones in Sixteen 188.8.131.52.