Thanks for your advice!

I try to make the pixel have "retro" feeling, to make contrast of the background, that's why they're very flat and have very large pixel, but maybe this idea doesn't work!!

( or it maybe work .....as very distinguish style?)

Whatever will be, I think my sprite still need some brush up! and I've come with the more clean-up(except the big guy) sprite. here:

Notice that the big guy is somewhat Bigger than he used to be in the mock-up screen. It's just because I don't how to reduce him!!! I use the image size in Photoshop to reduce him to 30%(because I made him x3 bigger, and forget to save the original small one!!

) And his pixel begin to out of the line!! (cannot explained, just see the pic below) I think there is no way to reduce him again, and I must work on new one, is that so?

Thanks for all!