Placing absolute behind relative positioned element

I’m not sure which one you want in front, but you just need to set position on both and set z-index on both.
http://jsfiddle.net/8eLJz/2/

a {
    color: black;
}

nav#mainNav {
    position: relative;
}

nav#mainNav > img {
    position: absolute;
    width: 100px;
    left: 0;
    z-index: 5;
}

nav#mainNav > a > img {
    width: 100%;
}

nav#mainNav > nav {
    width: 100%;
    position: relative;
    z-index: 10;
}

nav#mainNav > nav > a {
    display: block;
    text-align: right;
    background-color: yellow;
}