if (document.images) {  

image1on = new Image();  
image1on.src = "images/home_patient_on.gif";  

image2on = new Image();  
image2on.src = "images/home_medical_on.gif";  

image1off = new Image();  
image1off.src = "images/home_patient_off.gif";  

image2off = new Image();  
image2off.src = "images/home_medical_off.gif";  

}

function changeImages() {

  if (document.images) {

    for (var i=0; i<changeImages.arguments.length; i+=2) {

      document[changeImages.arguments[i]].src = eval(changeImages.arguments[i+1] + ".src");

    }

  }

}