take a look at astridsolcer.nl. at the very bottom we see an airplane. But i want the plane visible fully width.  I put the picture in a module and placed the module in that section. Whne i put the picture on the background in the section you cannt see the pic complete on a smartphone or ipad. I want to have it at the same look as the big picture at the very top

You need to set your section to "without container". You do this in the template options under Layout in the section. Then check the settings and remove any values ??(if present) for Margin and Padding for all display sizes. Please watch the videos, such as the one here: Layout:
www.joomlaplates.de/template-konfiguration/layout.html