Paint.net is amazing to use. It can be just as powerful as Photoshop. I think you might need to explore more into it though.
The first thing you should work on is your lighting on your render. I recommend getting a lightroom or even creating your own. Your render is pretty bland at the moment.
The second thing is that you shouldn’t make the render floating. You made the character’s foot touch the edge a little, but it still looks awkward. I recommend lowering the render.
The third thing is that you need to change the effects you use. Right now, the icon is just filled with white lines and sparkles. Maybe try a lens flare?
The forth recommendation is that you should change the background or the text. The reason why is because the text blends too much with the background and if you want the text to pop, you will need to change it.
Overall, it is not too bad but it just needs more work. Although you already tried out paint.net and become decent with it, I also suggest trying out Gimp.