a.cb-menu-button img {
    opacity: 0;
    transition: opacity 0.5s linear;
    -webkit-transition: opacity 0.5s linear;
    -moz-transition: opacity 0.5s linear;
    -o-transition: opacity 0.5s linear;
}
a.cb-menu-button:hover img {
    opacity: 1;
    transition: opacity 0.5s linear;
    -webkit-transition: opacity 0.5s linear;
    -moz-transition: opacity 0.5s linear;
    -o-transition: opacity 0.5s linear;
}