Background image does not "cover"

Hi! (says a newbie)

first of all: sorry for my rudimentary english.

I have some problems with a background image (Theme Tesserac, what is a nice theme).

On my Homepage a picture under the Header should be in full width, as well as on notebooks an mobiles (reponsive).
First I implement a Widget called “Visual Editor”. With the upper symbol “wrench” I defined in row Styles > Design > Background Display “Cover”.

– The Page in question is defined as ” Full Width Page”.
– via “Visual Editor” plus a “Spacer”-Plugin I defined the height as 723px (which is exactly the height of the picture). The original picture is 1500px width (what I did not defined by spacer).

In the end: The picture is notably clipped on the rihgt side; important parts of the picture are cutted off. And it is not responsive.

Where I made the mistake?

Clueless
Fritz