I have a placeholder background image that i want to replace after its loaded.
$.get("BIG-IMAGE-URL").done(function(data){
$('.MY-DIV-CLASS').css('background-image', 'url("BIG-IMAGE-URL")');
});
You can load your big image with javascript, and set the CSS background in the load event handler.
var img = new Image(); // Create new img element
img.addEventListener('load', function() {
// set background here
$('.MY-DIV-CLASS').css('background-image', 'url("bigImage.png")');
}, false);
img.src = 'bigImage.png'; // Set source path
Source: MDN Using_images
Additionally, here is a similar solution using jQuery How can I check if a background image is loaded?