<div class="wrap">
    <div class="thumbnail currentimg"></div>
    <div class="thumbnail"></div>
</div>
<div class="wrap">
    <div class="thumbnail"></div>
    <div class="thumbnail"></div>
</div>
<div class="wrap">
    <div class="thumbnail"></div>
    <div class="thumbnail"></div>
</div>
<div class="wrap">
    <div class="thumbnail"></div>
    <div class="thumbnail"></div>
</div>

<div class="next"></div>
<div class="previous"></div>

jQuery('.next').click(function() {
  var newImg = jQuery('.currentimg').next('.thumbnail').css('background-image');
  newImg = newImg.replace('url(','').replace(')','').replace(/"/gi, "");
  jQuery('.currentimg').next('.thumbnail').addClass('currentimg');
  jQuery('.thumbnail').removeClass('currentimg');
  jQuery('.featured-image').html('<img src="'+newImg+'"><div class="button prev"><span><</span><span><</span></div><div class="button next"><span>></span><span>></span></div>');
});