[Edi]

There is no prophetshape command. It's in the wishlist IIRC, but does not exist currently.

Modifying a targa file, you need an image processing program like GIMP or similar. See the mapmaking.pdf and the section on the image file (chapter 6, I think).

A basic guide to modding? Sorry, no such shortcuts. Read the modding.pdf in the docs folder. It has basic weapon modding, basic monster modding and armor modding as well as the nation modding and other sections. Read it, then look at some of the existing mods to see how they have been implemented in practice and then try to make your own simple mod.

Essentially, you have to do some work yourself to learn this stuff to be able to do it, but with the new manuals, it should not be difficult.