Image responsivity

I have a full width header gallery on my site. When I switch to different device, the images are either cropped or too big. Why aren’t they automatically responsive to different screen sizes?

I have one larger screen and one smaller notebook to work on travels, but the web site always looks different. You can see the different sizing in the pictures below. The image is used as background in a strip.


Basically I need the height of the strip/galerry to be responsive in the editor on different screen sizes.

Can somebody help me with this issue? Thank you.