Thanks for writing in!
You can add a revolution slider which will serve as you hero image inserted above your masthead. To know how you can integrate the slider to the theme, please check this out:
If you already have a slider and have integrated to the theme, please make sure that your slider is fullscreen. For the navigation to display right below the slider, you must add an offset height in the slider settings to leave a space for the menu navigation.
Full screen for your slider layout:
.masthead as offset:
Hope this helps.