The problem is that the program is paid. If png versions were on there too...Actually, the entire mod source is disponible on GitHub, you just need to look in the right place to find the sprites.
For example, the companion sprites are in Creatures folder, each folder is to a different companion (Terrarian companions will not have a sprites, obviously). Items... Well, on Items folder.
If you plan on trying improve the sprites, you will have to pay attention to where those sprites are placed on the companion, and do notice that some sprites, like Body Front and Right Arm Front doesn't exactly follow the other body parts spritesheet (that's to save virtual memory space).
The other thing, is that a sprite change may cause me problems in the future, even more when making new companions, but I could add them as an already unlocked skin to the companions.
Ps:. Also, Aseprite is awesome, I would have several problems splitting the companion body parts if wasn't for It.