Hi classact and welcome.

I'm very worried about your site. The answer to your question is that the Zen Cart banner is a background image which is defined in your stylesheet. But you have many stylesheets all doing the much the same things, so it's actually defined twice, as are many things. I'm not sure how you got into this position, but I am sure that this is going to be a big problem for you going forward.

Exactly where and how the header image it is defined depends upon your template. And that's another problem. Unfortunately, you have defaulted to the "Classic" template which is intended primarily for backward compatibility with earlier versions of Zen Cart and has been hacked about somewhat to achieve this making for a rather more complex stylesheet than a new site needs and more tables-based layout than is the case for a completely clean installation based on the template_default.

It would be better to build your own template from scratch based on the functionality in the template_default. A little more work up front, but much, much easier in the long-term.

If that seems a bit overwhelming, an alternative is to start from the Future Zen template which engages template_default for you and amend that. You can download it from the downloads section of this site and there's a link to the relevant downloads page in my signature below. This would get you back to a stylesheet where everything happens in one place.