Hi, @Sebastien27.
Thank you for your message, I would be happy to offer some suggestions for this. I think you would benefit from taking a look through our Uploading Images Guide, specifically the section that touches on wide images on tall screens: 
If I am not mistaken, our Brooklyn theme has a slideshow height setting called ‘Adapt to first image’ which seems to maintain the full image on mobile view. Since you are using a third-party theme, you might want to touch base with the theme developers to see if there is any setting you may have overlooked. The Kingdom theme was developed and is supported by a team called "Krown Themes" and you can reach out to them on their site here. Alternatively, you could use images where the focal point is in the center, as our guide above touches on.
I hope this points you in the right direction, feel free to reach back out here if you have any questions at all, we are always happy to chat!
