I tried that just now and my background disappeared. When I removed the center center fixed; it came back.
I accidentally stumbled across this code background-attachment: fixed; and it worked.
Yes I did. There is defiantly an extreme learning curve to this program.
My advice is to spend a weekend doing code tutorials first. In the long run, you'll be mighty glad you did .
If coding is not your thing and never will be, then you should probably be using Muse. It's aimed at hobbyists and designers who just need basic websites and have no coding skills.
For backgrounds it is always best to use longhand until you are comfortable coding. It is also a good idea because Dreamweaver is not nearly as friendly a CSS editor as Front Page was. Using shorthand will often leave you HAVING to code CSS manually as the so-called CSS Designer panel will drive you nuts. So, this would work:
background-position: whatever you want;
background-size: use cover if you want the image to fill the screen;