Header Image options

Dear Support, I am trying to manipulate somehow parameters and I need your assistance for two cases. www is: www.papapaj.pl I would like to adjust header image to be shown in full screen header part (without this additional square on the top of the website). How to do that? Additionally how to change a bit size of the image. I rather prefer to use less resolution like 1140 × 300 instead of 1140 × 450. second question is about background color under the main menu and footer, is it possible to change it, and where? Thanks in advance, Lukasz