if you want to create a store banner you would have to do it your self with a program like Photoshop or free alternative like Gimp, one you've made your own go to
1. store admin
2. design
3. banner and background then you just upload it
I've used the site several times, it's not overly complicated and free to use. My current blog banners I made at Picnik, but the site is closing on 4/19.