Alright, I'm no pro at spriting, but I have studied Metal Slug sprites for a while now. The ones you've made are pretty good, but here are some tips. Make the body more smaller, cartoonish. It looks nice, but it also looks to realistic. Also, use less antialiasing. Metal Slug hardly ever used antialiasing, sometimes a bit of dithering yes, but rarely antialiasing. Also, each color set like skin, hair, shirt, etc. was limited to four colors. The lightest shade where the light hit, the normal color, a darker shade, and an much darker shade for details or very dark areas. Hope this helps, and so far your doing a good job.