They randomly add extra art, and while the bonus art has a separate numbering so you could make the bonus art into it's own chapter somewhere, but that's honestly too much of a pain so I'll just throw all the art in to the last chapter I'm updating on that batch at that moment.
Edit: Made a compiled "chapter 0" with all the bonus art, but will still leave the art that's at the end of the chapters (currently 53 and 59) as they are now, as well as continue to include the art at the end of each batch I upload like I described, unless someone has a better idea or MD has a correct way of handling bonus art like this.