Fixes: Safari background doesn't display.

RIght now the index page banner doesn't display:

![no image](http://samsm.com/whatever/skitch/Atlanta_Ruby_Users__Group-20130605-175106.jpg)

Looks like Safari was being picky about (what seems to be a valid) background css declaration. Breaking out the background-size declaration fixed it on my local Safari. Hopefully does the same on mobile!