Why do you think its higher quality if you add a bigger image but then downscale it in-game?
An sprite of 128 x 128 looks good with an image that's 128 x 128, as good as an image that's 256 x 256. It may even look better due to down-sampling losing some details (blurry).
in gimp my paintings looks good but when i export it to C2 its looks blurred or with pixels and i dont know how to improve the quallity of the images.