Background image on body gets removed in aggregated file

This is a kick-ass module. Yet an issue with CSS background images plagues me.

On this site http://squaw.wonderlustfest.com/ I have a background image applied to the tag.

The CSS looks like this:

  background: url("http://9394bc4f934eb8c957d8-2f084e1f525b6270d41d6d2c79f4c609.r93.cf1.rackcdn.com/2013-images/site-bg.jpg") no-repeat fixed center top;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;  

The image is being served from a Rackspace CDN. Yet when I load the page and look at the body properties in Firebug, or the aggregated CSS file, there's no trace of the background image properties.

I have tried turning on bundling, async, flushing cache, rebuilding etc but no joy. Any experienced agg'ers out there have a clue?