I used ASEPRITE and it's FREE, the only problem is to install you just need to find the source code in GITHUB, there is a few instructions to follow. But i have to say has a huge difference compared to paint and others softwares for pixel art, Kiritia, PixelEdit... etc. I tested and Aseprite is the best.I saw people use PyxelEdit, it's free but idk about all the features it has