Betheme allows to set background images for both: headers and subheaders. As if that is not enough, you can set them individually for specific pages or the same for all.
To set same header background for all pages, please go to Betheme > Theme Options > Header & Subheader > Header > Background section. Here, you can upload custom background image, set its position or size. If you like to have individual header backgrounds, you can set them using Featured Image option located on the right hand side, while editing or creating pages.