1. This totally depends on what you’re trying to do. If you Google “learning CSS” you might find some good resources that will get you started.
2. Parallax can only work when there’s content and the image is a background image – regular images can’t have the parallax effect I’m afraid.
2b. You’re wanting to add an image above your categories? That’s kind of tough, but there is something information on this forum that may help:
3a. This is coming in the next version of GP Premium – we’re just finishing up some last tests before releasing it 🙂
3b. You can use Menu Plus for this. Go to “Customize > Menu Plus” and you’ll see the Menu Logo option.
Hope this helps!