Compress .png files losslessly.

Use zopfli-png with 1000 iterations, pngout /r with various blocksizes and deflopt.

apple-touch-icon-114x114-precomposed.png | Bin 865 -> 751 bytes
apple-touch-icon-144x144-precomposed.png | Bin 947 -> 939 bytes
apple-touch-icon-57x57-precomposed.png   | Bin 495 -> 450 bytes
apple-touch-icon-72x72-precomposed.png   | Bin 575 -> 512 bytes
apple-touch-icon-precomposed.png         | Bin 495 -> 450 bytes
apple-touch-icon.png                     | Bin 495 -> 450 bytes

The total difference is -320 bytes.