Use shorthand class .bg-{attachment} to control background attachment.
<div class="bg-gray-lighter bg-scroll bg-no-repeat" style="background-image:url(bg.jpeg);"></div>
<div class="bg-gray-lighter bg-fixed bg-no-repeat" style="background-image:url(bg.jpeg);"></div>
<div class="bg-gray-lighter bg-local bg-no-repeat" style="background-image:url(bg.jpeg);"></div>.bg-scroll
.bg-fixed
.bg-local