I try to stay away from using irfanview when reducing colors for tile inserting, it sometimes removes some of the colors you want and with manually reducing the colors yourself that doesn't happen. I personally prefer using paint alone but this method is a lot faster (and a bit harder). anyways good tutorial, I'll have to try it myself sometime.
(I accidentally made this post on lepug's thread thinking it was this thread :P)
EDIT: Here's an image of a house tile with the colors reduced to 16.
The house on the left is the house that I reduced the colors with on paint (manually) and the house on the right is the one I used irfanview to reduce the colors. They both have the same amount of colors but the one on the left has a lot more detail on the roof than the one on the right.