Both color and image can be set as a section background. If you choose an image, you can also change its overlay opacity 🌔
Within the editor, click on the section you want to edit and select Change background:
Choose between Color and Image:
If you go for image background, 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
Your changes are saved automatically but don't forget to update your website for the changes to reflect online.
NOTE: It is not recommended to use a GIF for a section background because it may affect the loading speed of your page