function preloadImages(){
var t='<layer top="-100" left="0" visibility="hide"><div style="position:absolute; top:-100px; left:0px; visibility:hidden">';
t+='<img src="http://www.unionlearn.org.uk/images/structural/ltu/tab-s.gif" alt=" " height="10" width="10" />';
t+='<img src="http://www.unionlearn.org.uk/images/structural/ras/tab-s.gif" alt=" " height="10" width="10" />';
t+='<img src="http://www.unionlearn.org.uk/images/structural/raot/tab-s.gif" alt=" " height="10" width="10" />';
t+='</div></layer>';
}

// Off images
ltu_off = new Image;
ltu_off.src = "http://www.unionlearn.org.uk/images/structural/ltu/tab.gif";
ras_off = new Image;
ras_off.src = "http://www.unionlearn.org.uk/images/structural/ras/tab.gif";
raot_off = new Image;
raot_off.src = "http://www.unionlearn.org.uk/images/structural/raot/tab.gif";

// On images
ltu_on = new Image;
ltu_on.src = "http://www.unionlearn.org.uk/images/structural/ltu/tab-r.gif";
ras_on = new Image;
ras_on.src = "http://www.unionlearn.org.uk/images/structural/ras/tab-r.gif";
raot_on = new Image;
raot_on.src = "http://www.unionlearn.org.uk/images/structural/raot/tab-r.gif";

function button_on(imgname) {
  if(document.images) {
    document.images[imgname].src = eval(imgname + "_on.src");
  }
}

function button_off(imgname) {
  if(document.images) {
    document.images[imgname].src = eval(imgname + "_off.src");
  }
}