demo

$('<img>').load(function(){
    // make sure to bind the load event BEFORE setting the "src"
    alert('image loaded');
}).attr('src',function(){
    var imgUrl = $('body').css('background-image');
    imgUrl = imgUrl .substring(4,imgUrl .length-1);
    return imgUrl;
}).each(function() {
    // fail-safe for cached images which sometimes don't trigger "load" events
    if(this.complete) $(this).load();
});