1. webmaster mode is your friend
http://help.x-cart.com/index.php?title=X-Cart:Using_Webmaster_mode
2. you have learned that header images are oftentimes controlled in css -- but oftentimes they are controlled in templates, such as head.tpl
for example, in my skin (loosely based on books & magazines), my header image lives in the template, but controlled by css
/skin/books_and_magazines/customer/head.tpl
USE WEBMASTER MODE and you will learn much about where things live