I used jpegcompressor.com and saw a 50-60% size reduction in most PNG files, much better than C3’s built-in compression. I even tested it by compressing all the images from my game (though it was a bit tedious since the tool only allows 20 files at a time).
Results:
C3 compression: 4.8 MB
jpegcompressor.com: 2.1 MB
This makes a big difference, especially for web or mobile games. Could C3 include a similar compression method?