function navigation_onMouseOver(sender) {
    if(!sender.originalSrc) sender.originalSrc = sender.src;
    
    var newSrc = sender.src;
    newSrc = newSrc.replace('.on.png', '.png'); // prevent doubling of ".on" tag if it's already there
    newSrc = newSrc.replace('.png', '.on.png');
    if(sender.src != newSrc)
        sender.src = newSrc;
}

function navigation_onMouseOut(sender) {
    if(sender.originalSrc)
        sender.src = sender.originalSrc;
    else {
        var newSrc = sender.src;
        newSrc = newSrc.replace('.on.png', '.png');
        sender.src = newSrc;
    }
}