If you have chosen a Theme for your site, you won´t be able to change the background without going into the bowels of FrontPage, finding that theme and replacing the background image with one of your own choosing...

If you are creating a site from scratch, however, inserting a background image is done in the same way as you would have done for PowerPoint (Format/Background/....)

When deciding on your background the crucial principle is:

Quick download!

Research shows that speed of download is the single most important factor in getting and keeping your readers on the web.

We hasten to add, however, that the design of your site MUST be determined by your client´s needs, and the target audience. If you are targeting teenage computer enthusiasts, you will probably have to create a dark brooding black-background/red text site with eerie repetitive music and oversized graphics which take half an hour to download.

However, for the rest of humankind, create backgrounds which contain simple (or no) background graphics. And don´t forget to create thumbnails, and resize graphics where possible, to speed things up even further.