The site builder has its own image tool that helps you search and pull from a free (to use) source of images, resize your selected image and insert it into the template.
To get started, select a block that you'd like to edit the background image for:
Then in the right hand style editor, click the 'Open Image Library' button:
Here, you'll have the option to either upload your own images from your local computer, or search the gallery with a keyword. Try putting in 'dog':
Select an image you'd like to use and the cropping tool will pop up for your to make any last minute changes to your image:
Then hit "select" and the image will be processed by the server. After processing you can click 'Apply Changes' and your image will be successfully added to your website.