Setting a default thumbnail url if no thumbnail

Just set up a condition to check that thumb_id isn’t empty. If it is, use your alternative image. Pretty simple stuff.

    <?php
$thumb_id = get_post_thumbnail_id();

if ( $thumb_id ) {
$thumb_url = wp_get_attachment_image_src($thumb_id,'thumbnail', false);
$thumb_url = $thumb_url[0];
} else {
$thumb_url="http://www.website.com/img/default.jpg";
}
?>

<div class="content" style="background-image:url(<?php echo $thumb_url; ?>);">