Portrait Quality

Hello friends. I would ask some advice for you. I'm swapping Portraits of my Chars, but note that they are not getting the same quality of earlier because when I convert to 256 colors, they lose a lot of color. However, in other Portraits I see are images rich in color and detail but no failure. Is there any tips or different from putting good portraits maintaining a good quality way?

I suggest you to work with png format, pics in bmp format lose a lot of quality when swapping to 256 colors
and I strongly suggest you to use a good photo editing software like gimp (it's also easy to use and freeware).
I'm looking forward to see your first works