Problem with background images in mobile version

I’m having some trouble getting the mobile version of a website to respond to the image sizes as it should. When I inspect the website in Chrome the desktop and mobile versions look ok, but when I look at it on my phone, the images are way too big, as if they aren’t responding to the size of the screen at all.

Here’s the Codepen of it, and here’s the Github repo. I’d appreciate any feedback!

Desktop screenshot:

Inspector responsive screenshot:

Mobile screenshot:

After posting this on Stackoverflow as well I think the solution was to not make the background fixed.