For that, please follow following steps:
1. Goto Appearance=> Customize=> Header Image => Enable Featured Header Image on
2. Make sure Excluding Homepage, Page/Post Featured Image is selected. You can also select other options depending on your need. Visit this link for more detailed instructions on what each option does.
After this, your featured image for posts/pages should show up as header image like in demo. If there is no featured image, then default header image will show up.
If the image still does not show up, then go to the edit page of that page/post, then look for Fabulous Fluid Options and make sure Default option is selected in Header Featured Image Options
Let me know if you have further problems and also, please post in your Site URL.
Regards,
Pratik