Those are background images stored in the images folder of the TEMPLATE you are using.
In a fresh install, the template displayed is " classic ".
You FIRST need to create your own template (on the forum it will be referred to as CUSTOM).
What you are seeing is probably:
includes/templates/classic/images/tile_back.gif
You need to create your own template, and then create your own background image called tile_back.gif
So you will end up with:
includes/templates/CUSTOM/images/tile_back.gif
You don't have to use tile_back.gif ...
The background can be styled with any image, or any HEX colour and the declarations for it are all in stylesheet.css
To create your own template, look at:-
https://www.zen-cart.com/tutorials/index.php?category=4