$(document).ready(function() {
  var img1 = $('#lightboxlink1').attr('href');
  var img2 = $('#lightboxlink2').attr('href');

    
  $.ajax({
    url:img2,
    type:'HEAD',
    error: function()
    {
        //file not exists
        //alert('only one image');
        $('#lightboxlink1').attr("rel", "lightboxalt");
        $('#lightboxlink2').attr('rel', 'lightbox');
    },
    success: function()
    {
        //file exists
        $('#lightboxlink1').attr('rel', 'lightbox');
        $('#lightboxlink2').attr('rel', 'lightbox');
    }
  });
});