Hello
Sorry to say all I can suggest is to use the following Custom CSS which will display your background image in it's original size. It is simply impossible for your image to fill every screen size without some degree of resizing/cropping.
#container_header {
background-size: auto auto !important;
}
Or even the following which will reduce your image so all of it can be displayed with no cropping however as with the above solution you will be left with some white space on certain screensizes...
#container_header {
background-size: contain !important;
}
Alternatively you could try resizing your image (extend height) so the width will be less likely to be cropped on any given screen size.
Ciarán