Here is a small snippet to change your background-color into a great image.

Just like this: Hover me

To do that, disable the general background of your app and optionally disable the header background:

add the following css in the “advanced customization” section of your editor:

.pane {
background-image: url("https://i.imgur.com/jyeOuJh.jpg");
background-size: cover;
}

Obviously you have to change the url code “http://i.imgur.com/jyeOuJh.jpg” for your own.

To get the best result, choose an image with a portrait format.

Important information:
That modification changes all of yours features’ backgrounds.