Hi! Sorry for the late reply, as I don't get notifications for this game. I'm the lead artist (Pete design and animations, logo, main menu, UI and events)
I used Fire Alpaca for all of my art and animations! It's not the best for animations, as it doesn't do vector art, so the quality suffers a bit, if you transform too much. But it's free and really user friendly, so it's a really good starting point, or even a continuing point if you can't buy a fancier program and don't want to pirate.
Hope that helped!