function preloadImages() {
  if (document.images) {

    aon = new Image(); 
    aon.src = "images/navbutton_a_on.jpg";
    aoff = new Image(); 
    aoff.src = "images/navbutton_a_off.jpg"; 

    bon = new Image(); 
    bon.src = "images/navbutton_b_on.jpg";
    boff = new Image(); 
    boff.src = "images/navbutton_b_off.jpg"; 

    con = new Image(); 
    con.src = "images/navbutton_c_on.jpg";
    coff = new Image(); 
    coff.src = "images/navbutton_c_off.jpg"; 

    don = new Image(); 
    don.src = "images/navbutton_d_on.jpg";
    doff = new Image(); 
    doff.src = "images/navbutton_d_off.jpg"; 

    eon = new Image(); 
    eon.src = "images/navbutton_e_on.jpg";
    eoff = new Image(); 
    eoff.src = "images/navbutton_e_off.jpg"; 

    fon = new Image(); 
    fon.src = "images/navbutton_f_on.jpg";
    foff = new Image(); 
    foff.src = "images/navbutton_f_off.jpg"; 

    gon = new Image(); 
    gon.src = "images/navbutton_g_on.jpg";
    goff = new Image(); 
    goff.src = "images/navbutton_g_off.jpg"; 

    hon = new Image(); 
    hon.src = "images/navbutton_h_on.jpg";
    hoff = new Image(); 
    hoff.src = "images/navbutton_h_off.jpg"; 

    ion = new Image(); 
    ion.src = "images/navbutton_i_on.jpg";
    ioff = new Image(); 
    ioff.src = "images/navbutton_i_off.jpg"; 

    jon = new Image(); 
    jon.src = "images/navbutton_j_on.jpg";
    joff = new Image(); 
    joff.src = "images/navbutton_j_off.jpg"; 

  }
}

function imgOn(imgName) {
  if (document.images) {
    document[imgName].src = "images/navbutton_" + imgName + "_on.jpg";
  }
}

function imgOff(imgName) {
  if (document.images) {
    document[imgName].src = "images/navbutton_" + imgName + "_off.jpg";
  }
}  

onLoad=preloadImages();