Ok guys... in the past week I have had 4 separate community managers ask me the best way to switch out a background image in both Ning 2.0 and 3.0 networks. Here is the way that I do it.
This method will also double as a pageview tracker using goo.gl URL shortener.
The CSS you'll need for the perfect full width background image is as follows:
background: url(http://goo.gl/OQQeTw) no-repeat top center fixed;
*don't forget to change the image link in red with your own
The process is simply to 1) upload an image through a blog post 2) shorten the link for that image url and 3) snap it into place.
Here is a little animation for exactly how to do this:
Let me know if you have any further questions or need help with your Ning development. Enjoy!