if (document.images) {

patientsup = new Image();
patientsup.src = "http://www.nevadasleep.com/assets/images/hnav/patients.jpg";
patientsdown = new Image();
patientsdown.src = "http://www.nevadasleep.com/assets/images/hnav/patients-over.jpg";
companyup = new Image();
companyup.src = "http://www.nevadasleep.com/assets/images/hnav/company.jpg";
companydown = new Image();
companydown.src = "http://www.nevadasleep.com/assets/images/hnav/company-over.jpg";
sleeplinkup = new Image();
sleeplinkup.src = "http://www.nevadasleep.com/assets/images/hnav/sleep.jpg";
sleeplinkdown = new Image();
sleeplinkdown.src = "http://www.nevadasleep.com/assets/images/hnav/sleep-over.jpg";
communityup = new Image();
communityup.src = "http://www.nevadasleep.com/assets/images/hnav/cpap.jpg";
communitydown = new Image();
communitydown.src = "http://www.nevadasleep.com/assets/images/hnav/cpap-over.jpg";
mainofficeup = new Image();
mainofficeup.src = "http://www.nevadasleep.com/assets/images/hnav/main.jpg";
mainofficedown = new Image();
mainofficedown.src = "http://www.nevadasleep.com/assets/images/hnav/main-over.jpg";
hendersonup = new Image();
hendersonup.src = "http://www.nevadasleep.com/assets/images/hnav/henderson.jpg";
hendersondown = new Image();
hendersondown.src = "http://www.nevadasleep.com/assets/images/hnav/henderson-over.jpg";
summerlinup = new Image();
summerlinup.src = "http://www.nevadasleep.com/assets/images/hnav/summerlin.jpg";
summerlindown = new Image();
summerlindown.src = "http://www.nevadasleep.com/assets/images/hnav/summerlin-over.jpg";
pahrumpup = new Image();
pahrumpup.src = "http://www.nevadasleep.com/assets/images/hnav/pahrump.jpg";
pahrumpdown = new Image();
pahrumpdown.src = "http://www.nevadasleep.com/assets/images/hnav/pahrump-over.jpg";
renoup = new Image();
renoup.src = "http://www.nevadasleep.com/assets/images/hnav/reno.jpg";
renodown = new Image();
renodown.src = "http://www.nevadasleep.com/assets/images/hnav/reno-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" );
    }
}