Just add this code to your image css

body{
 background:
        /* top, transparent black, faked with gradient */ 
        linear-gradient(
          rgba(0, 0, 0, 0.7), 
          rgba(0, 0, 0, 0.7)
        ),
        /* bottom, image */
        url(http://fc02.deviantart.net/fs71/i/2011/274/6/f/ocean__sky__stars__and_you_by_muddymelly-d4bg1ub.png);
    }

Reference: linear-gradient() - CSS | MDN

UPDATE: Not all browsers support RGBa, so you should have a 'fallback color'. This color will be most likely be solid (fully opaque) ex:background:rgb(96, 96, 96). Refer to this blog for RGBa browser support.