You can just use paint. It's not too bad acually, I've done one charset titled 'girl in bikini' It's not all that complicated just do trial and error when messing with a naked sprite.
I can probly do it because I'm semi talented with paint. I used to make cd covers, my friends thought they almost looked profesonal too. too bad I don't have them saved any more. My mom was stupid and destroyed the comp last year, after I got it running again everything I had saved was gone. Now my mom isn't allowed to touch the computer.
But back to the subject, try using paint. It's mainly trail and error.