Hey there,
I use Photoshop, but I’m aware it’s not as readily accessible (and not really worth it unless you plan on making money through it in some way), but GIMP is one of the closest.
The downside to things like Pixlr and Paint.net is that it’s internet based and that can cause problems in terms of lag and quality. I definitely recommend GIMP since it’ll give you the best quality. It is a bit more difficult to navigate, but I suggest kind of looking things up as you go. The way I taught myself Photoshop is simply by googling things like “how to make transparent png photoshop”. I’m sure you’re more than capable of that since Episode coding can take similar amounts of effort to learn.
In short, GIMP is harder to learn but worth it. You just need to do it in small steps.
Also I’m not sure what phone/tablet you have, but a lot of great free apps can actually be better than some computer apps. A lot of big Episode editors use Apps! Some examples I can think of are Medibang, Ibis PaintX, and Superimpose.
Anyways, I hope that helps at all. 