PNGs were optimized using several tools in sequentially: TruePNG, pngwolf, leanify, advpng, ECT, deflopt, defluff. This optimizes the filters, removes any unused data, optimizes the color palette and optimizes the deflate compression. The whole process is completely lossless.
JPEGs were processed with jpegoptim to optimize the huffmann headers. Again, completely lossless except for most metadata which has been stripped (some JPEGs contained exposure and photoshop correction information).
Niklas Fiekas