The backstretch call was wrong.
Instead of this:
$.backstretch(images[index], {speed: 500});
I wanted this:
$.backstretch(images, {speed: 500});
$('body').data('backstretch').pause();
It's not that the next/prev buttons weren't working, it's that the initial call was passing the first image, not the set of images.
The second line (w/ pause in it) is there so the images don't change automatically, they only change when I hit the next/prev buttons.