<div>
 //my blog post
</div>

<div style="background-image:url("image.jpg"); opacity:0.5;">
<div style="opacity:1;">
 //my blog post
</div>
</div>

<div class="container">
    <div class="content">//my blog post</div>
</div>​

.container {  position: relative; }

.container:before {
    content: "";
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    z-index: 1;
    background-image: url('image.jpg');
   opacity: 0.5;
}

.content {
    position: relative;
    z-index: 2;
}​