I dontknow how to post images directly, but here is a link to my fileshare with 5 images in. Bungie.net : Reach : Player File Share
The link has everything you need. If you don't do it, your thread will be locked. As a side note, I wouldn't even post a map until you're sure it's the final version. Either way, take the time to do it right.
*500 letter Talk about your inspiration, ideas, goal and reason for the map. The Map looks cool, what is the custom gametype?
A description is NOT required, and your thread will NOT be locked if you do not post a description of your map. It is, however, another great chance to show off your map and let people know about it. One of the most important things is a list of the weapons and vehicles which will give people an idea of what type of gameplay to expect.