Create a new outline, assign it to the menu item you want a different background image on. Then go into the Style tab for that outline and click on the check box that allows you to edit the background image. Now you will change it to the one you desire for this menu item / outline.
You can then inherit each section you do wish to be the same as the Base Outline using that sections gear icon and make choice to either inherit or clone from Base Outline. You can read more of that on the Gantry website.
Another way to set backgrounds to a section without using different outlines is to create a Page Class for that menu item... Example:
Menus > Main Menu > Home > Page Display tab > Page Class and I'd type the word home in the box. Which creates the page Class for CSS as .home
Then in your Custom CSS file Joomla Root > templates > rt_helium > custom > scss > custom.css file (create scss folder and custom.css file if you haven't already) you can call and target that Page and change or add any background you like to any section of that page using CSS background-image: ; and or background-color: ; as well.
.home #g-showcase {
background-image:url("images/backgrounds/my-image.jpg");
background-size: cover;
background-repeat: no-repeat;
}
So in this example above I am first calling the page Class .home followed by the section I wish to target. In this case I am targeting the ID g-showcase with #g-showcase
So that gives you two solid options to choose from.
Best of luck...
Grant / WebTread