Firstly lets address the issue of background images. To start, I would NOT recommend that you use a MOVING background image. They can be very distracting and put people off your site completely. Even still images need to be used carfully and very subtly (look at this site), and often just a solid colour is best.
That said there are several ways of loading a background image, but for now we can use the following:
This will set an image called 'background.gif' in your folder 'images' as your background image. If the image you want to use is called something else then, obviously, just change 'background.gif' to whaterver your image is called.