$("ul.nav li");
ul li {
    display: inline-block;
}
ul li img {
    opacity: 1;
    -webkit-transition: opacity .5s;
    -moz-transition: opacity .5s;
    transition: opacity .5s;
}
ul:hover li img {
    opacity: .5;
}
ul:hover li img:hover {
    opacity: 1;
}