It is necessary for the site "administrator" to have a fundamental understanding of how HTML works within the framework of a PHP site.
The CONTENT of the main page (as with all other defined pages and EZPages) is nothing more than "raw" html.
This raw html is entered into the system (by most users) in the ADMIN >>> TOOLS >>> DEFINE PAGES EDITOR interface.
Here, zencart comes packaged with a very basic WYSIWYG html editor called "HTMLArea". It's a rudimentary editor, and a bit "clunky" but you can still construct a basic content layout using this editor.
While it IS possible to "compose" your page content in an external editor - even MS FrontPage - there are a number of important "rules" that must be adhered to when using an external editor.
The first "rule" is that when you have composed your page content, you use COPY & PASTE to bring the raw html into the system. You need to use the <> toggle icon to change HTML area to Source Code mode, so that you can "paste" the html into the input window (field).
The second is that you copy and paste code that exists ONLY between the <body> and </body> tags. DO NOT post any header code.
This third rule is to know how to manually edit the PATH references to the <img src=....> tags. (You will need to specify the path to your relevant images, based on where these images are located on your SERVER (not on your home computer hard drive). You will also use RELATIVE and not ABSOLUTE paths.
If you are starting to feel a little lost from what I say above, then you need to do some basic tutorials on HTML coding, and a good place to start is https://www.w3schools.com.