Thanks this helped but there are still some issues with black bars.
For me it would be sufficient if i do not have an header background image at all but i still want to have an image as the top content over the whole screen width and beginning under the nav bar. Do you have a tipp on how i can achieve this. (Remove header image – ok, create an image block on startsite page – ok (but this does not fit over whole screen width and begins not under the navbar but on top of page)
Thanks indeed this works but, it is the same as setting position:relative.
The black bars above and down are visible.
