function showThumbs() {
  document.getElementById("photoThumbs").style.display='block';
  document.getElementById("photoSlideshow").style.display='none';
}

function showSlideshow(mainPictureName, captionText) {
  document.getElementById("mainImage").src = mainPictureName;
  document.getElementById("caption").textContent = captionText;
  document.getElementById("photoThumbs").style.display='none';
  document.getElementById("photoSlideshow").style.display='block';
}