Until now, it has been quite easy to create PNGs of drawings for my RPG tokens, especially in Photopea, which has an automatic Magic Wand feature, so I just touch them up afterwards.
However, I am having difficulties with this particular artwork (https://files.catbox.moe/fn016v.jpg), and I have no idea why. So, I realize that I need to learn more about how to create PNGs in general.
What are the different approaches to making PNGs? If possible, links to tutorials/blogs would be very helpful.