The config editor contians a tool that allows you to upload graphics and use them in your config.
You will download a sample background from this wiki, navigate to the media manager, and upload it there.
Click the house-icon in the upper left corner to return to the main menu.
From the main menu, navigate to the media manager.

Click the Browse...-button to select the downloaded background from your disk to upload.

Then, select a name for the image (this name is only visible here, and only meant for managing your media).
Finally, click Upload.
The uploaded image will appear like this.

Clicking the Copy URL-button copies the url to the image into the clipboard. This url can be used as a background.
Note that the url only includes the suffix, not the prefix (https://url/) is not included, so the link does not work itself, the app fills in the prefix instead. This is done to allow for the image to exist on multiple servers.