Paint.net is what I used I'm gonna give GIMP a try later, is there a pokemon you want to request for my next one?
Well it is pretty complicated but that just means it has more features haha Paint.net is a slightly less complex one if you wanna try it.
I'm not to good at using GIMP my first time resulted in me having to go through 5 different tutorials to get all the toolbars back XD but I might try some pixel art on there