Event.observe(document, "keydown", function(e) {
  // 37 for left, 39 for right
  var cur = location.href;
  var prev = $$("#image-nav-prevlink a")[0].href;
  var next = $$("#image-nav-nextlink a")[0].href;
  switch(e.keyCode) {
    case 37:
      if(cur != prev) { location.href = prev; }
      break;
    case 39:
      if(cur != next) { location.href = next; }
      break;
  }
});