Kirejara - Like I said above the Minkowski Cage wasn't indexed at all, it was still in RGB (16.7 mil colors) - Civ can only display properly indexed images. However, the previous pcx, Berninghaus Chair, was properly indexed so there shouldn't have been a problem with that one (Don't know why it was showing black for you guys). As I said before DPI really has no relevance to Civ - it's generally used to determine the quality of an image if it's "printed" (Often the default resolution in graphics programs is set to 300dpi for printing and 72dpi for display). When indexing you're compressing the number of colors into a set amount (256 for Civ) and storing that info in a palette. As far as I'm aware Irfanview cannot index an image directly, but I believe you can import a palette and make adjustments to the color array.
Theov - I use Photoshop, although I'm familiar with GIMP. What kind of issue exactly are you having with GIMP? That guide I linked above from Kyriakos is fairly comprehensive for basic palette work.