In subtheming Commerce Kickstart, how to load my own background image?
It's necessary to go in /admin/appearance/settings for your custom sub-theme and in vertical menu on the left, click "Toogle Styles", uncheck the box for "Commerce kickstart theme global style" and click "Save configuration". Now click again in "Toogle Styles" and there is a new option: "MY THEME NAME theme global style. (all) - MYTHEMENAME_style.css".

Check this option and again click in "Save configuration".
Now changes in attributes in mysite/sites/all/themes/MYTHEMENAME/css/MYTHEMENAME_style.css does work.