Most of the Divi options can be optimized for better user experience on small screens, however, there are some options that cannot be defined separately for small screens, like the Background Image option.
Landscape oriented images may look great on Desktop but on small screens they might be cropped because of the different aspect ratio.
In this case, you can simply define a new module with a different mobile-optimized background image.
You can define portrait oriented images for Mobile and landscape oriented images for Desktop. You can also define a different one for Tablets.
All you need to check is Divi's Visibility options under the module settings (Advanced tab) where you want to define a different image.
This technique allows you to create an additional module and configure it the way that you want for a specific mobile device (Tablet or Phone).
Tip: You can use the device view option from the page settings bar to preview your changes.
Note: Modules that are hidden on a device will display at 50% opacity in the Visual Builder.