You can change that background for all pages/posts (except custom home pages which are assumed to have sliders at top) by going to Theme Options->Backgrounds tab and setting the following option –
https://docs.google.com/file/d/0B9j6FQo4cSQFZDVSYjFmeE9sLUU/edit?usp=drivesdk
This will override the default image but leave any custom header you have set on a individual post/page basis intact (as in the about us page in demo site). Thanks