I don't usually do do digital art, but I gave it a go for the first time. The programme I used is Paint.net. I'd like some constructive criticism, how could I improve blah blah.
Like @Sinister already said, it's a nice approach but you're being restricted by the programme you are using. It may be worth investing a little in to a better software for what you do.
However, it's nice work you have on the shading/shadowing (especially for paint). Since this is just a first attempt, may be have a few more tries and share them with us. Nice work, well done.