Image problem header, masthead, col-full

[ Moderator note: moved to Fixing WordPress. Please do not use Developing With WordPress for these topics. ]

Good day to all.

Here is a problem I can not solve.
I want to display a background image in the banner. It must be 1950X500.
It’s good when I’m at 100% level display …
But when I get down at least 30%, well we see very clearly the background image that reproduces to the left and right of the Masthead. This is not the case for other sites.
I just want it to be limited to its size of 1950X500 which corresponds to the col-full part of theme.

The masthead includes the col-full and the Before and After part how to modify the element so that Before and After remains empty! Transparent … in all cases that it no longer displays a part of my image that I want set in col-full only.

<img src="https://logic-68consolessystem.fr/wp-content/uploads/2017/04/Réparation-Console-la-Vitrine-Logic-68-Logic-68-Consoles-System-Mozilla-Firefox_2.jpg" alt="screener" />

Thank you I really hurt, I rummage in storefront I find the tag, but do not understand how to achieve what I do not want to see anymore.

Thank you for your help