you put the banner easly! ^_^
go to any page you want to advertise your story (for example as i do in my profile)
just press add image and down where you can control what size you want your picture to be up line writes link! so copy the link of your story and paste it there! :)
after when you reload the page as normal to view if you press on the banner it goes to your story! :)
i hope i helped you ^~^
if you have questions just tell me! i will try to explain you again and again until you understand ♥
i was the same as you like 3 months ago or 2. So slowly you will find out everything ;)

If you go to your story and edit the foreword, there should be an "Add Images" below. Just click on that and you have the option to add the main image and the background with the picture's link.
I hope that helps. :)