First of all, you have to make them in B&W. Then when you change them to index mode/256 colors, you need a
B&W palette and then switch that palette into the LvL3 Portraits that PotS, Jmorphman, etc have (or even the ones I make).
Just in case you don't know how to do it, here I leave you that
palette.
You can send me a PM and I can help you better so we don't
destroy this topic. I'd like to see your attempts