The background for the body image is called from
template_css.css - line 8
background: url("../images/s5_background.jpg") no-repeat scroll center top transparent !important;
So basically what you need to do is download this image here:http://www.shape5.com/demo/touch_of_soul/ex3//templates/touch_of_soul/images/s5_background.jpg
And edit it the exact way you want. Then upload and overwrite the existing image called s5_background.jpg in the /images folder on your FTP or File Manager for your website.
Make sure you make a backup before overwriting anything.