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;
}