clone img src to use as a div background via jquery

I'm having trouble trying to clone the string of a URL in the src= value of an . I'm trying to define the string as a variable so that I can append the "background-image" css value with said variable. ... Taking an and using it as a background-image in a

I'm doing so in a $(window).load event since I think the has to be rendered in the DOM in order for it's URL to be present. However, nothing seems to be working. I can alert(imgURL); but fail to plug it into the 's background.

Here's what I have so far. Any help would be greatly appreciated. Thanks!

$(window).load(function() {
  var imgURL = $('.thumbnail img').each(function() {
    $(this).attr('src');
  });
  $('.fullsize').each(function() {
    $(this).css('background-image', 'url(' + imgURL + ')');
  });

});

.fullsize {
  width: 200px;
  height: 200px;
}
.thumbnail img {
  width: 75px;
  height: 75px;
  float: left;
  cursor: pointer;
}

<div>
  <div class="fullsize"></div>
  <div class="thumbnail">
    <img src="http://ift.tt/1KPPEIo" />
  </div>
</div>

<div>
  <div class="fullsize"></div>
  <div class="thumbnail">
    <img src="http://ift.tt/1RMfGBX" />
  </div>
</div>