var imageURLs = $('div');
imageURLs.each(function(index, element){
    var imageURL = $(element).css('background-image').replace('url("', '').replace('")', '');
    if (imageURL != "none"){
        $.ajax({
           url: imageURL,
           type: 'HEAD',
           error: function(){
              //error handling for broken image url
           }
        });
    }
});