So usually you use <body bgcolor="#xxxxxx"> to specify the color, right?
This time, instead, use <body background="imagename.jpg">
It's probably best to keep a background color, so until the background image loads, the visitor to the page sees the color you intended. This is especially true if you plan to use a dark background image and light text. If you don't specify the background color too, they won't see the text until your image loads! So you can use:
<body background="imagename.jpg" bgcolor="#xxxxxx">
PS - the background image will tile by default (so if it is small, you'll see them repeat over and over again all over the screen). I don't remember how to stop this using straight HTML, but if you want more information I can explain how to do it with CSS.