Have you experimented with using the CSS filter (currently only functional in webkit)?
Take a look at this resource: http://davidwalsh.name/css-filters
(I haven't had the opportunity to test it myself, but you may need to separate the image and the filter into two separate divs)
Apologies for any language errors :)
EDIT:
I've created this pen: http://codepen.io/anon/pen/gEmrD
HTML:
<div class="container">
<div class="content">
</div>
<div class="form">
<p>This represents your form (use your imagination!)</p>
</div>
</div>
CSS:
.container {
width: 1000px;
height: 100%;
min-height: 500px;
background-image: url('http://www.gordonviaggi.it/files/wedding_packets/in_giro_per_san_francisco.jpeg');
background-size: 1000px;
background-position: 50% 50%;
display: block;
}
.content {
width: 100%;
height: 300px;
margin: 0;
background-image: url('http://www.gordonviaggi.it/files/wedding_packets/in_giro_per_san_francisco.jpeg');
background-size: 1000px;
background-position: 50% 0;
filter: blur(3px);
-webkit-filter: blur(3px);
display: block;
}
.form {
position: absolute;
top: 200px;
left: 250px;
width: 500px;
height: auto;
background-color: rgba(255,255,255,0.5);
color: #222;
display: block;
}