Why shouldn't I use black background wallpaper?
Black backgrounds can have dramatic effects for display, but for visitors with laser printers who want to print your web page, the black background will create gray text (instead of inverse white on black) and will be difficult to read.

How do I stop visitors from listing my graphics directory?
Make sure every directory has an index.htm or index.html file that will at the least block a default directory listing. Ideally, use meta tag to automatically route visitors with newer browsers to your home page. For example:
<META HTTP-EQUIV="Refresh" CONTENT="0;URL=http://www.bigzeus.com">