I was wanting to have a Bootstrap/Bootswatch well with a background image instead of dropping the image inside, using css to jam it in in the right shape (rounded corners, etc) and trying to overlay the content in that. This was how I got around it. I suspect there's a better way of doing it (especially something with responsive images) but it's…

HTMLsnippet

<div class="well well-lg" data-well-bg="path/to/image">
  Content and stuff.
</div>

well-bg.js

jQuery(document).ready(function($) {
  $('.well').each(function () {
    var data  = $(this).data('well-bg');
    if (data) {
      $(this).css('background-image', 'url(' + data + ')');
    }
  });
});