If you have an element with a background image, like this
You can wait for the background to load by getting the image URL and using it for an image object in javascript with an onload handler
var src = $('#test').css('background-image');
var url = src.match(/\((.*?)\)/)[1].replace(/('|")/g,'');
var img = new Image();
img.onload = function() {
alert('image loaded');
}
img.src = url;
if (img.complete) img.onload();