Making a map flooded is a little complicated for anyone who's knew, but basically you can use the Map Editor (look around the forum for more information on this).
To save it as a desktop, and by desktop I think you mean background, you will have to take a screenshot of what you want as your background in-game. To do this you have to press the key on your keyboard that says "prt sc" or "print screen." What this button does is takes a picture of whatever is on your computer screen and puts it in your copy clipboard, making it so you can paste it in say, microsoft paint, by pressing ctrl + v. Once the image is pasted, just save it and name it whatever you want. After the image is saved, go to it and right click it. An option for "Set as background" should come up, click this.
Hope this helps
