script.js

$(function() {
  $("div.content").live("showoff:show", function(evt) {
    var bg_img = $("img[alt=background]", evt.target);
    if (bg_img.size() > 0) {
      var src = bg_img.attr("src");
      bg_img.hide();
      
      // Set new background on body
      $("body").css("background", "url(" + src + ") no-repeat center center fixed")
              .css("-webkit-background-size", "cover")
              .css("-moz-background-size", "cover")
              .css("-o-background-size", "cover")
              .css("background-size", "cover");
    } else {
      $("body").css("background-image", "");
    }
  });
});

style.css

#preso, .slide {
  background-color: transparent !important; 
  border: none !important;
}