Banner Image always cut off. How can i change it?

Site URL: https://lisarettenbacher.com/

Hello everyone, i hope i find help here. I have tried a few things already but nothing seems to work.

Currently I have one banner image, but I would like to add an image collage of three images (all in one image) so I can showcase more images at once. 

I added a few sample images. The photo with the couple is my current banner, but I would like to change it to the one where you can see three images in a row. Unfortunately the banner always gets cut off so the images aren't displayed properly. Is there any way to change the banner width to always stay at a 100% without scaling. I would like this to work on mobile devices as well of course.

I have a block identifier and tried to change it in the code but i couldn't make it work. 

I use the theme "Sonora" (Brine Family)

Thanks for helping!