Hi Harald,
Thanks for reaching out.
You have two options, first is adding it through CSS, example, add this to your Theme Options > CSS with the URL of image background.
header.x-masthead {
background-image: url(http://example.com/example.jpg);
background-size: cover;
}
With this setup, the background color of header bars should be transparent. Else, it’s not gonna work.
The second option is if your header bars are sticky. With that, you have to add the same image for each header bar’s background. BUT, you have to change the background position of the second image equal to the height (but negative value) of first header bar. This creates an illusion of single background image which in reality, two backgrounds.
First Header bar


Second Header bar

The downside with this is when the header bar shrinks, then two background images will not going to sync properly.
Thanks!