Hey Joao,
By "fit’ I believe you want all parts of your image to show. That possible only if, the Aspect Ratio your image used as a background matches the aspect ratio of the section.
This issue is usually seen on portrait phone screen orientation because commonly, users use landscape image background. If you view that background in a portrait screen, the sides of that landscape background would be cropped because the background covers the section or container. Containing the background is not good either as you’ll see blank spaces around the image.
The only solution if you wish to display the desired parts of the image prepare an image for each aspect ratio of the section per screen size. If you notice, your section when viewed on a desktop is usually a long horizontal rectangle. If you resize your browser, it becomes a short horizontal rectangle. On a portrait phone screen, it becomes a long vertical rectangle. That means you to prepare an image for each of those aspect ratios.
If you’re displaying an image only and want to show all parts of it, I’d recommend using an Image element instead. You can make it span full width by disabling the Row’s Global Container.
Hope that helps.