On several sub-pages we used this same background image for SubHeader section. This image you can find here: images/demo/subheader.jpg  - in your QuickStart package. You can simply replace image with a new one. Default dimension of .jpg file used in demo was 1920 × 399 px.

But if you want to use different image for different sub-pages we suggest to upload and use image by changing settings here, in Menu Manager for each menu item.