Hi xentryx
It due to the image ratio and the space of that section, when you reduce the screensize, the image will be cropped to fit the height + width of the small screen.
You can show the full image by changing: background-size: cover;
to: background-size: contain;
However, the height of the image will be reduce (to show full width of image) and you will need to edit the height of that block on small screen accordingly.
Regards