A page can have a background, which can be an image, a single color, or another website.
For this example, you will use the image you uploaded earlier.
The url of it should still be in your clipboard. If not, return to the media manager and copy it.
Background image - URL/Color-field.Type-field, select image (It's a dropdown menu that activates once you click on it).1180 pixels wide.665 pixels tall.
Save-icon in the upper left cornerPreview-button next to it. This will open a new window, and show what the device screen would look like with the resolutions of this new window. Feel free to resize this window, and see how the background behaves.Editing the config in multiple browser-tabs is strongly discouraged. Opening a config in a tab stores its current state, and editing/saving it in another tab does not refresh the first tab.
In essence, doing this will cause you two have to separate version of the same config, each overwriting the other when clicking on 'Save' in its tab.
It is, however, perfectly fine to use a second tab for read-only-purposes, e.g. to go to another config and see how they specified something.