var imageUrl = "images/sky.jpg";
$(".div_with_class").css("background-image", "url(" + imageUrl + ")");
$('myObject').css('background-image', 'url(' + imageUrl + ')');
jQuery('selector').css('background-image'); // gets the image URL
jQuery('selector').css('background-image', 'path/to/image.jpg'); // sets the image URL