last = 
       
          first = 1;          
          current = 1;

          function nextPicture() {
              // Hide current picture
              object = document.getElementById('slide' + current);
              object.style.display = 'none';

              // Show next picture, if last, loop back to front
              if (current == last) { current = 1; }
              else { current++ }
              object = document.getElementById('slide' + current);
              object.style.display = 'block';
          }

          function previousPicture() {
              // Hide current picture
              object = document.getElementById('slide' + current);
              object.style.display = 'none';

              if (current == first) { current = last; }
              else { current--; }
              object = document.getElementById('slide' + current);
              object.style.display = 'block';
          }