I'm not sure that you will get too many answers because most of us picked up this stuff on our own by reading the existing code. The documentation is a good way get a general understanding of how the system works, however, it can't cover everything. You fill in the holes by looking at the code and experimenting.
That said, I'll take a shot at answering your questions.
1) If the background picture never changes, then you can probably keep the picture in the /web/openacs-4/www/ directory and reference it directly in the default-master.adp file. Otherwise, you can probably stuff it into the content repository.
2) Assuming that you read the basic documentation about permission, ACS 4.0 Permissions Tediously Explained is good at filling in the details.
3) You can check to see if you are the site admin (ad_permission_p -3 admin). I think -3 is the default_context. I could be wrong. Anyway, in your adp template, you can put a if statement around the HTML code that shows you the original page so that only the site admin can see it.
4) Edit the default-master.adp file. That changes the site wide look.
5) I have no idea. I use vi to write HTML code 😊
6) You can compile and activate nscgi. I think someone wrote up some info about nscgi. I don't know where it is.
