You can use a container for your background, placed as absolute and negative z-index : http://jsfiddle.net/2YW7g/
HTML
<div class="main">
<div class="bg">
</div>
Hello World!!!!
</div>
CSS
.main{
width:400px;
height:400px;
position:relative;
color:red;
background-color:transparent;
font-size:18px;
}
.main .bg{
position:absolute;
width:400px;
height:400px;
background-image:url("http://fc02.deviantart.net/fs71/i/2011/274/6/f/ocean__sky__stars__and_you_by_muddymelly-d4bg1ub.png");
z-index:-1;
}
.main:hover .bg{
opacity:0.5;
}