Forum:How can i create a banner?

i want to create an image that will be on top of the screen ( i guess its called "banner"); just like that sea and statue image on top of that link:

http://lostpedia.wikia.com/wiki/Main_Page

so how can i insert an image like this, and make it background image? i don't know anything about page coding, so i hope someone can tell it easy like telling to a dumb -_-"