var defaultimage = new Image();
defaultimage.src = "/css/images/ball-gray.png";

var overimage = new Image();
overimage.src = "/css/images/ball-yellow-small.png";

$(document).ready(function () {
    var links = $("#nav a");

    links.mouseover(function () {
        $(this).children("img").attr("src", overimage.src);
    });

    links.mouseout(function () {
        $(this).children("img").attr("src", defaultimage.src);
    });
});