Hi Samuel Cawood,
Thanks for upgrading to pro version. Here is the step to add banner image just below the menu heade areas.
First go to dashboard -> appearance -> widgets -> on the right you will get widgets areas called Banner , there you need to drag and drop text widgets
Then on text widgets content area you need to add this image tag:

Note: After uploading the required banner image via media gallery then for the image url
Go to dashboard -> media -> click on the required image -> on the right most you will URL field then copy whole link and paste on to src property.
Example:

Let us know if you got any problems on this, we will be more than glad to assist you.
For the reference: @SEE
https://codex.wordpress.org/WordPress_Widgets
https://codex.wordpress.org/Media_Library_Screen
Thanks