Header images sticky to top - Where to add padding?

Hi,
my header imageis sticky to the top, but I want to leave an empty space between the top and the header (maybe 20 or 30 px). If i try to change the header padding on Customizing > Layout > Header, the image remains glued to the top and now I have a white space below the image. I want to do exactly the reverse, image glued down to the menu and empty space in the top.

Maybe I have to write something on the field "position" but I don't find instructions.

Thanks for any help

Pietro