The banner background of the pages is given with CSS. So, in general, you have to change the banner image with CSS. But you can also change it with helix3 - for that go to Menus >>Main Menu. Now select the page you want to change.

Now in the menu tab go the option "helix page title".

Now add the background image in the background image option. As you see you can change here also Page Title displayed above image.