[Endoperez]

About maps: any picture can be a map. However, the game only reads one thing out of them: the color white (255,255,255). Every white pixel is a province. The provinces are numbered a spesific way which gives them numbers. The province numbers won't change unless white (=new provinces) is added or removed from the tga.
The neighbourhood is determined with #neighbour pr1 pr2 tags in the .map file. You have to determine the neighbouring provinces manually. However, the game also reads red or mostly red colors (I haven't checked this out myself) as borders. That gives a starting point, but presumably isn't good enough in most cases.

Yes, only 17 nations can exist. This will change for DomIII, which will allow (much?) higher number of nations.

I don't know about any real guide for modding, but looking at some complete mods should help you out.

And finally, the game doesn't have that kind of a save&load command. When you Save&Exit, only the commands you gave for that turn are saved. Only files that a game "save" has is the situation of the whole world, nations, etc in fthrlnd, the part of the situation a spesific nation has (jotun.trn, machaka.trn, abysia.trn...) and the commands human player has given to a nation (jotun.2h, machaka.2h, abysia.2h...). Previous turns aren't saved.

However, if you or an automated program makes a copy of the game directory, it can be just pasted back and it works like traditional "save". There is a Java program which saves & reloads Dominions games, but I haven't used it and don't remember its name.