onError = null;

     if (document.images) { 
img1on = new Image();  
img1on.src = "about_on.gif";
img2on = new Image();
img2on.src = "contact_on.gif";
img3on = new Image();
img3on.src = "links_on.gif";
img4on = new Image();
img4on.src = "mission_on.gif";


img1off = new Image();  
img1off.src = "about_off.gif";
img2off = new Image();
img2off.src = "contact_off.gif";
img3off = new Image();
img3off.src = "links_off.gif";
img4off = new Image();
img4off.src = "mission_off.gif";

}

function imgOn(imgName) {
if (document.images) {
document[imgName].src = eval (imgName + "on.src");
}
}
function imgOff(imgName) {
if (document.images) {
document[imgName].src = eval (imgName + "off.src");
} }