Placing absolute behind relative positioned element

I have two elements in the same container, the first has position: absolute, the second position: relative. Is there a way to set the "z-index" of an absolute element so that its in the background? The relative positioned element should be on the top (z-layer) because it holds links...

Here is a JSFiddle: http://jsfiddle.net/8eLJz/

The absolute positioned element is on the top of the z-layer and I have no influence with the z-index-property. What can I do?

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>

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/