I have tried to set the img.bg to position: absolute; http://jsfiddle.net/piggybattery/9QzmS/2/ but then it gives a horizontal scrollbar.
It does matter if the image matches the background color of the body.
I rather have a fit for the common resolutions 1366×768 and 1920×1080 then a strange distorted.
By example if I view a background image form 1024x 683 at a viewport 1280×868 it displays a nice proportion
But with a viewport of 1920×858