function roller(id,source,which,FileType) //rollover function for images
{
    var _end = (which == 1)? '_on' : '_off';
    var _FileType = (FileType == 'jpg')? '.jpg' : '.gif';
    var elem = document.getElementById(id);
    if (source == '')
        elem.src = 'images/' + id + _end + _FileType;
    else
        elem.src = 'images/' + source + '/' + id + _end + _FileType;
    
}