[cflsystems]

In 4.4 things are al ittle bit different then earlier versions. You will find everything related to csutomer side in "skins/common files" and then in "skins/skin files" the template that are different for each skin. By your reply above I'm starting to think you don't know css. It is kinda a requirement to know css/html and smarty to be able to edit templates in xcart. Take some time to learn at least the basics (www.w3schools.com is a good place to start).
You have to find the css class used for the header and apply "background" property to it which will hold the image, something like this

.header_class_name {
background: url(path_to_image/image_name);
}