var arrayImages = new Array(2);
arrayImages[0] = "../images/GoButton.png";
arrayImages[1] = "../images/GoButtonHover.png";

for (i = 0; i < arrayImages.length; i++) {
    var preload = new Image();
    preload.src = arrayImages[i];
}

function imgGo(imgIndex) {
    document.getElementById("GoButton").src = arrayImages[imgIndex];
}