Within the builder, click on the section you want to edit and select Change background:
Choose between color or image:
Color background: click on the color bubble and set any color you like.
Image background: click Add image or Replace image and upload your own image or browse the free image gallery. After setting a background image, you can also:
Change the overlay opacity until you're happy with how dark or light your section's background is
Enable fixed background which means that the background image will remain fixed (won't scroll) when scrolling the page
These changes are saved automatically, but remember to update your website for them to reflect online.
NOTE: It is not recommended to use GIFs for a section background because it may slow down your page