use firebug with FireFox to troubleshoot css issues. see the attached screen shot
fire bug reports that the image cannot be loaded.
remember the path for the iamge needs to be realtive to the CSS file. the css files you define this in is 2 levels deep:
WA_eCart/CSS/eC_Clean_Pacifica_Arial_Custom.css
so your path needs to be relative to that file:
background: url(../../energy-saving-light-bulbs/btn-basket-checkoutoff.png);
or use a site relative path:
background: url(/energy-saving-light-bulbs/btn-basket-checkoutoff.png);