Actually..
Actually a map Preview can be 128x128 as well. You should use that way It works a lot better and It makes you map smaller
Okay and to stay on topic this is how you add a description.
First open world Editor then open your map.
Now at the top bar you'll see a bunch of stuff like file, advance, edit, tools, etc. Anyway you want the one called scenario. Then click on the first button called Map Description. Theres where you fill out what the game is called. Now to skip 5 hours of me talking to you guiding you step by step. I'll cut the crap and just answer you question. Go to Loading Screen and hit the "Use Campain Screen" Button. Then in the text below type what ever you want because thats what is going to show up in the loading process.
Then you can pick a loading screen from the list (my fav is undead final bu I don't use those anymore because I made my own).
And there you have it. If you have anymore questions just post in this form and I'll try to answer them (because everyone else is to buissy to help someone out *shame*)