Hi Markus,

This will be possible with some CSS. Please follow these steps:

1. Navigate to the media tab, upload the background image and copy the URL of the image

2. Then go to Pages and hover the page link (don't click to edit). In the bottom of your browser you will see a URL slug. In that slug there is a post=40 or post=72 (for example). That number is the page ID you need (screenshot)

3. Then navigate to Appearance > Customize  > Theme Options > Custom CSS and add the following to the CSS textarea:

body.page-id-921 .content {
   margin-top: -50px;
   padding-top: 50px;
   background-image: url(your-image-url);
}

Please change the 921 ID to your page ID and add the background image URL instead of 'your-image-url'.

That will do the trick.

If you have any questions, feel free to ask.

You're welcome!