function increment(listid) {
  return changeImage(listid, 1);
}
function decrement(listid) {
  return changeImage(listid, -1);
}
function changeImage(listid, change) {
  if (document.getElementById) {
    // Update selectednumber
    slideshow = window.slideshows['list_'+listid];
    arraylength = slideshow.length/2;
    selected = window.slideshows['selected_'+listid];
    selected += change;
    if (selected<0) {
      selected = arraylength - 1;
    } else if (selected >= arraylength) {
      selected = 0
    }
    window.slideshows['selected_'+listid] = selected;

    slideshowelement = document.getElementById('slideshow_'+listid);
    if (slideshowelement) {
      // Update image
      image = document.getElementById('listimage_'+listid);
      if (image){
        image.src = slideshow[selected*2];
        // Update count
        span = document.getElementById('listcount_'+listid);
        if (span) {
          //XXX: InnherHTML is nonstandard
          span.innerHTML = (selected+1) + '/' + arraylength;

          // Update text
          textfield = document.getElementById('listtext_'+listid);
          if (textfield) {
            textfield.innerHTML = slideshow[ selected*2 + 1];
            return false;
          }
        }
      }
    }
  }
  return true;
}