Store builder does not change the look or header or background of your website.

Store builder simply inserts products into your existing websites only.

Think of it as you already have your website built, wether it be a full html template, php or whatever. The store builder code is then placed on a page within that existing website.

That is part of the reason your not finding information for how to add headers, images, backgrounds and stuff to store builder. It in itself is a tool to add to existing websites, Not for creating a website.

Now a days tons of people are using wordpress because of its simplicity, thousands of templates you can find around or make your own and great tools like the store builder helper plugin you can find at http://return-true.com/2009/03/zstore-helper-wordpress-plugin-for-zazzle-store-builder/