Oh, no wonder why they look jagged and pixel-y you ARE making these with pixels in MS Paint. At first I assumed you were using MS Paint but some of these look to clean to be from that. Really, with work this good you really should try working in more powerful programs to make your art look as good as it can.
There is a 100% free program called The Gimp that a lot of artists recommend using if you don't want to spend anything. I've only played in it a bit, but I think you really deserve to work outside of MS Paint because you are so good at doing this.
Excellent work, regardless.