function roll_over(img_name, img_src)
   {
   document[img_name].src = img_src;
   }
function changeImage(obj) {
//check to see if we've just clicked the object that has already been clicked
if (lastClicked && lastClicked.src == obj.src) {
return false;
}
//change the last clicked item back to it's original state
if (lastClicked) {
lastClicked.src = lastClicked.src.replace(/\_over/, "");
}
//set the lastClicked image to one currently clicked
lastClicked = obj;
obj.src = obj.src.replace(/\.gif/, "_over.gif");
}