if (document.images) {
homelinkup = new Image();
homelinkup.src = "http://www.nevadasleep.com/assets/images/vnav/home.jpg";
homelinkdown = new Image();
homelinkdown.src = "http://www.nevadasleep.com/assets/images/vnav/home-over.jpg";
patientsup = new Image();
patientsup.src = "http://www.nevadasleep.com/assets/images/vnav/patients.jpg";
patientsdown = new Image();
patientsdown.src = "http://www.nevadasleep.com/assets/images/vnav/patients-over.jpg";
companyup = new Image();
companyup.src = "http://www.nevadasleep.com/assets/images/vnav/company.jpg";
companydown = new Image();
companydown.src = "http://www.nevadasleep.com/assets/images/vnav/company-over.jpg";
sleeplinkup = new Image();
sleeplinkup.src = "http://www.nevadasleep.com/assets/images/vnav/sleep.jpg";
sleeplinkdown = new Image();
sleeplinkdown.src = "http://www.nevadasleep.com/assets/images/vnav/sleep-over.jpg";
communityup = new Image();
communityup.src = "http://www.nevadasleep.com/assets/images/vnav/cpap.jpg";
communitydown = new Image();
communitydown.src = "http://www.nevadasleep.com/assets/images/vnav/cpap-over.jpg";

}

<!-- repeat the 4 lines above for any subsequent images. -->
function buttondown(buttonname) {
    if (document.images) {
      document[buttonname].src = eval(buttonname + "down.src" );
    }
}
function buttonup ( buttonname )
{
    if (document.images) {
      document[buttonname].src = eval(buttonname + "up.src" );
    }
}