function change_image()
{document.about.src = "images/aboutOn.gif"}
function normal_image()
{document.about.src = "images/aboutOff.gif"}
<!-- work -->
function change_image2()
{document.services.src = "images/servicesOn.gif"}
function normal_image2()
{document.services.src = "images/servicesOff.gif"}
<!-- contact -->
function change_image3()
{document.contact.src = "images/contactOn.gif"}
function normal_image3()
{document.contact.src = "images/contactOff.gif"}