For a background image, people usually choose something that tiles nicely round the page. You then put into the BODY tag background= with the file name of the background image. There are many many images that can be used from the web, but you should only use ones which are free from copyright restrictions.
If you have a dark background, as on this other page, you will need to use a light text colour.
It is also essential that you use low-contrast images for backgrounds, otherwise the writing, light or dark, will be difficult or impossible to read. The two shown here are designed as backgrounds and have a reasonably low level of contrast.
You can save the background from Netscape by clicking the right mouse button while the cursor is over the background, and no other image.