Right now I have my CSS in:
/web/css/ directory.
It's also what Symfony Demo app uses.
Where should I store additional images used by CSS?
Simply in /web/images/ or maybe in my src/AppBundle/something?
I notice some other files in /web/bundles/name/ - should I move everything there?
I'm sorry it may look like a simple and stupid question, but it's confusing...
Place it wherever you want it and wherever it can be loaded by the browser.
Normally, web/ would be your server root. This means that only things in the web/ directory can be accessed by the browser.
How you name the directories, subdirectories, files, etc. is all up to you. Make a nice choice :)