So is photoshop if you can get your hands on a serial number online...anything is free if it has a serial number.
But MS paint is a great desighner. It's easy, It's fun, Usefull, good outcomes
But the only thing is that if you don't know how to use it proffesionally (lol) then only use it for pizelisation or sprite editing.
Gif ex is a very good program for .gifs if you want to make a moving sprite...
I mean take a look at this - http://www.youtube.com/watch?v=uk2sPl_Z7ZU
lol
PS is the best but you can sprite in Gimp just as easilly as in paint AND you can keep the BG transparent.i got my PS leggaly for 109 quid, Amazon FTW !
Like everyone else has said, MS Paint is a very poor program to use for complex artwork. However, there are ways to create nice pieces of artwork in it - you just have to know a few techniques.