As I recall, the Man Store template is a Template Monster free template, and TM can change things around from normal zencart behavior. A link or text address to your site may help.
But normally background images will go in to your templates/YOURTemplate/images folder.
You can set the background in the stylesheet, but since TM may have several stylesheets it can be uncertain where it is.
You can download any number of free templates that have background images to see how it is done. But here is the code for the All Business template:
body {margin: 0;font-family: verdana, arial, helvetica, sans-serif;font-size: 62.5%;color: #444444;background: #edf9fb url(../images/bk.jpg) repeat-x;}
You may need just the last part from "url" pertaining to the image. Name your image what you have called it.
Make sure that your background image is small and optimized for fast loading.