gistfile1.js

var images_to_captions = {
  "pic1.jpg" : "This is pic one",
  "pic2.jpg" : "second pic is here",
  "pic3.jpg" : "blargh", 
}

$.BgSwitcher.defineEffect("fadeAndSwitchText", function($el) {
  $el.animate({opacity: 0}, this.config.duration, this.config.easing, function(){
     // new slide so switch text here
     var currentimage = '?';// get this from the css or the context or something
     var caption = images_to_captions[currentimage];
     $('#caption-box-id').text(caption);
  });
});

$(".box").bgswitcher({
  images: ["pic1.jpg", "pic2.jpg", "pic3.jpg"],
  effect: "fadeAndSwitchText"
});