the code fixes the background in the center (vertical and horizontal). You may also check this: bg-position and bg-attachment. This site is really helpful by learning something about CSS and other code-stuff.
I was happy building my first website with Sydney too It was great!
I don’t know the plugin exactly, but normally you should be able to go into the plugin section from the custom css plugin and paste the code in it. It should work, yes.
I checked your link and this website doesn’t seem to have different pictures in mobile-portrait or mobile-landscape mode, but it does have indeed different images for the mobile version. Maybe it’s a feature in the slider the theme is using. You can try “Slider Revolution” instead of our theme slider, maybe it has the feature.
I don’t know whether it will come in an update - I guess Vlad and the other have to decide this. With some css-tricks you could have different slider images for mobile versions. This could be done with Media Queries, to set different background-images for different viewport sizes.
I just took a look at your website and the slider images on mobile. Sure, it’s not the best, but sometimes you can’t make it perfect to every viewport. And your images didn’t look ugly at all. It’s important that everything is readable, that the menu is responsive and something like this - sometimes you have to make a cut with the responsiveness I think.
Maybe try to reduce the slider height for mobile versions. Look at this topics to get the code: