snippet.js

$(function() {
  function pushState() {
    var url = $(this).attr('href');
    replaceImage(url);
    if (history.pushState) {
      history.pushState({}, '', url);
    }
    return false;
  }

  function attachHandlers() {
    $('a.previous-link, a.next-link')
      .unbind('click', pushState)
      .bind('click', pushState);
  }

  function replaceImage(url) {
    $('.body').load(url + ' .body', attachHandlers);
    $('.photo').css('opacity', 0.5); // Loading indicator ;)
  }

  $(window).bind('popstate', function(e) {
    if (e.originalEvent.state) {
      replaceImage(location.href);
    }
  });

  $(document).keydown(function(e){
    if (e.shiftKey || e.metaKey || e.ctrlKey || e.altKey || e.srcElement !== document.body) return;
    if (e.keyCode == 37) $('a.previous-link').eq(0).click();
    else if (e.keyCode == 39) $('a.next-link').eq(0).click();
  });

  attachHandlers();
});

Replace the .body selector with the fragment of the page that you want to replace.