<div class="parallax_bg"
data-top-bottom="transform: translate3d(0px, 40%, 0px);"
data-top="transform: translate3d(0px, 0%, 0px);"
style="background-image: url(http://bla-bla-bla.com/Cover-01.jpg); ">
</div>
var newBg = 'my.jpg'
var elem = document.querySelector('#parallax_bg')
elem.style.backgroundImage = 'url(' + newBg + ')'