function changeIcon() {
    var old = $(this).attr('src');
    var hover = old.replace("1", "2");
    if (old == hover) {
        var hover = old.replace("2", "1");
    }
    $(this).attr('src', hover);
}
$('document').ready(function() {
    $('nav ul li a img').mouseover(changeIcon);
    $('nav ul li a img').mouseout(changeIcon);
});