You're mixing apples and oranges, for understandable reasons.
1) Images are per site, and stored in Home/wp-content/uploads/sites/NUMBER
2) Themes and plugins are SHARED between all sites, and live like they always do in Home/wp-content/themes and Home/wp-content/plugins
HOWEVER. The same way that Home/sitename is virtual, WP calls the location of these things in a 'relative' way. So if I go to domain.com/Home/sitename/ it will pull images from domain.com/Home/sitename/wp-content/uploads/sites/NUMBER and so on and so forth.
This is the basis of a virtual sites :) It's okay. This is how it's supposed to work.
At another site the imaeg is:
That means a site specific image is loaded.
Where is that code if not in a site specific CSS?
Actually that entirely depends on the theme. If your theme has a way to upload background images and headers, then it stores them in that site's media library, the exact same way it does for single install of WP. :) Nothing changed here.
So back to this question:
I cannot find the directory for the language specific stuff.
At the /Home directory on the server I cannot find any directory named "english" or any other landguage root. I only fund wp-admin, wp-content etc and a few files.
There is no folder for the language specific directory, it's 100% virtual.
There IS a folder for per site images.
I think the "Ads" about multilingual sites talks about the advantage of having site specific CSS files so I presumed this was the case. I do however not want it at all - I want one and only one CSS file using exact the same images. How do I fix that?
No idea what you're talking about with 'Ads' but in general, you would need to install a plugin to handle per site CSS (I use JetPack which has a Custom CSS plugin). Alternately, you can put the images shared for all sites in the theme directly (well, a child theme - you'd never edit a theme directly, right? Right) or the plugin you create to do whatever this 'ad' thing is :)