Why not call it a submarine? I don't think that in alternate histories all things would change name. Massive ship, not the most effective use for a submarine, as the Japanese found out in World War II. I like.
Try paint.net and GIMP 2. Both are free, much much much better than MS Paint and very easy to handle. Both are good for painting and GIMP 2 creates quite interessting effects on your pictures. I use it frequently.