Z-index: -1;

HTML

<div id="parent">
    <div id="child">Child</div>
</div>

CSS

#parent { 
    background:#006DC2;
    opacity: 0.5;
    -ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=50)";
    filter: alpha(opacity=50);
    padding:20px;
}
#child { 
    background:url(http://avt.foto.mail.ru/bk/ae_design/_avatar) no-repeat;
    width:67px;
    height:90px;
    position:relative;
    z-index:-1;
}