Placing absolute behind relative positioned element

CSS has a z-index property so on your nav#mainNav > img selector just set z-index: -1;. Here is a working jsFiddle: http://jsfiddle.net/8eLJz/1/

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;
}

You can put a negative z-index on the image, which will cause it to layer behind the other elements:

a {
  color: black;
}

nav#mainNav {
  position: relative;
}

nav#mainNav>img {
  position: absolute;
  width: 100px;
  left: 0;
  z-index: -1; /* <----------------------------------- HERE I AM! */
}

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

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

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

<nav id="mainNav">
  <img src="http://www.colourbox.com/preview/7389458-682747-example-stamp.jpg" />
  <nav> <a href="/">Some Text</a>
    <a href="/">Some Text</a>
  </nav>
</nav>