$(document).ready(function() {
  var randomImages = ['1','2','3','4','5','6','7','8','9','10','11','12','13','14','15','16','17','18','19','20','21','22','23','24','25','26','27','28','29','30','31','32','33','34','35','36','37','38','39','40','41','42','43','44','45','46','47','48','49'];
  var rndNum = Math.floor(Math.random() * randomImages.length);
  $("#topImageContainer").css({ background: "url(images/random_bg/" + randomImages[rndNum] + ".jpg) 100% 100% no-repeat" });
});