Since you’re looking for free software have you ever used blender.org? It’s a cool software that people recommend a lot of special rendering 3D modeling, especially when creating character rendering such as GFX. This is a great software to use in my opinion.
Paint.net - Gimp I do see people have recommended it a lot. You could start learning some new things that are for beginners here like this post for example paint.net tutorial. It does seem to have some good tutorials on youtube and the forum, it’s actually easy to use if you practice and learn, and it’s free. It takes some basic understanding to produce good graphics with these programs.
Overall, I don’t have that much to say since there are similar topic to this one already exists on the forums. I recommend doing some quick research before creating a new thread.
This thread has a couple of free programs that could be used for (creating or editing) 2D PFP/ thumbnail GFX/ icons ect.