if you look at the faction.pcx (and faction2.pcx, faction3.pcx) files, you'll see that everything is well labeled, and you just have to paste in your new pictures.
The only important note is that these files use color tables, and you need to stick to the SMAC color tables. Photoshop can take any picture and force it into SMAC's color table...
BUT you also need to note that Color 0 is the transparent color, and colors 1-16 are NOT the same between SMAC and SMAX, which is why some things look a little different. No matter what you do, you need to make sure you stick to the SMAC or SMAX color table, and don't use color 0 for anything except the transparent parts.
If you want a nice template for making walls around your cities, go here:
http://www.crosswinds.net/~smaccf/
(under Faction Utilities / Graphics Overview)
