Map is just a plain picture (2048x1024x32 PNG) and a small text file map.cfg containing theme name to use. That's all about creating maps (well, there's a style guide here: http://hedgewars.org/node/704). You may find many samples in our hedgewars installation folder.

can you give me an example for the map.cfg?

You may see an example of map.cfg here:
http://fireforge.net/scm/viewvc.php/*checkout*/trunk/share/hedgewars/Data/Maps/mushrooms/map.cfg?revision=907&root=hedgewars

so in the map.cfg is just the name?

no, its the name of the theme

For example, I have made a map called Bubbleflow, and the map.cfg file contains the word "Underwater", so the game uses the Underwater theme alongside my map. Try it out!