var extention = ".gif";
var highlightSuffix = "_h";

function getHighlightedImageURL(imageURL) {
    return imageURL.substring(0, imageURL.length - extention.length) +
           highlightSuffix + extention;
}

function getNormalImageURL(imageURL) {
    return imageURL.substring(0, imageURL.length - extention.length - 
                                                  highlightSuffix.length) +
           extention;
}

function highlightImage(id) {
    var imageSrc = document[id].src;
    var highlightedURL = getHighlightedImageURL(imageSrc);
    document[id].src = highlightedURL;
    return true;
}

function normalImage(id) {
    var imageSrc = document[id].src;
    var normalURL = getNormalImageURL(imageSrc);
    document[id].src = normalURL;
    return true;    
}